Add "Include pugixml.cpp" and "Header Only" linkage options

The loop traverses the source tree and simultaneously builds up a copy
of it at destination. Short of race conditions, this code is safe -
however, it's not obvious that dit stays inside the destination tree.

This change adds a few assertions to help enforce/document these
invariants. One particular subtlety is that dit can actually *become*
null after we exit out of the loop, but it's guaranteed to only do so
once sit goes back to sn.

This is only possible when doing a full document copy - for some reason
we weren't using this for that (in reset(xml_document)), but we are now.

Fixes #314.

We were previously relying on non-standard comment detection that is
supported by gcc/clang to avoid warnings about implicit fallthrough.

This can be solved using attributes but using them requires a lot of
compiler-specific detection logic because not all versions of gcc/clang
support them.

We don't *really* need to rely on fallthrough here - the type conversion
block can be located *after* the AST type switch instead, which means
that any AST type that has type ambiguity can fall back to that in the
future.

Fixes #331.

Update CMakeLists.txt to rename the static and shared libraries to pugixml

Don't call them pugixml-shared or pugixml-static, because this breaks pkgconfig calls and userspace tools.

This makes sure all conversions work as expected (note that no type can
be converted to node set so we don't check that).

Add POSITION_INDEPENDENT_CODE option

Allow to use the static library in other dynamic librairies. This is a standard cmake option https://cmake.org/cmake/help/v3.11/prop_tgt/POSITION_INDEPENDENT_CODE.html#prop_tgt:POSITION_INDEPENDENT_CODE

Instead of performing a late null check that is redundant and only
needed to silence clang static analysis warning, we pick the context as
a root / self node. This way the code is a bit less redundant and the
static analyzer is happy.

Fixing possible null pointer dereference

explicit casts for -Wconversions warnings in gcc

Fix incorrect installation location for exported CMake targets

Fix install locations for pkgconfig *and* cmake

Modernize CMake

The block of code can be removed in the future if CMake 3.15 ever
becomes the minimum

Move pugixml-config.cmake.in to scripts/ to prevent polluting the root

New Features:

pugixml-shared and pugixml-static are "always" available, but not always
built. This allows downstream projects to still use them, and to make
sure our configurations are correct regardless of if they are being
built. They are not always installed however.

pugixml-shared and pugixml-static have ALIAS libraries of
pugixml::shared and pugixml::static respectively. These names are also
respected in the `find_package(pugixml CONFIG)` generated files, so its
safe to use pugixml in a CMake project regardless of whether it is
installed locally, or if its a subproject via `add_subdirectory`.

pugixml will automatically select the correct library type based on
BUILD_SHARED_LIBS. A pugixml::pugixml ALIAS is also available.

CMAKE_MSVC_RUNTIME_LIBRARY from CMake 3.15 has been backported.

CMake will now rely on generator expressions (a powerful abstraction in
large projects with many subdirectories) for most work. This offloads
work from the single-threaded configure stage to the multithreaded
generation stage.

pugixml now uses CTest as the runner.

Some settings are automatically disabled if pugixml is used as a
subdirectory. These are still able to be manually set, but are hidden
from folks who choose to use CMake GUI.

 Bump CMake minimum to 3.4
 pugixml no longer requires a C compiler to be found or set to
compile correctly. This speeds up configuration and building on windows.
 Begin laying groundwork to backport MSVC_RUNTIME_LIBRARY
property

Fixes #285 

This keeps src/ folder clean of auxiliary files only required for
special builds; note that CMakeLists.txt already depends on scripts/
(specifically for pkgconfig setup).

Closes #294 

The newly added tests make sure that during node/attribute destruction
we deallocate a few memory pages; this makes sure that we don't read
node data after it's being destroyed.

Also clean up formatting/style in the remove_* implementation a bit.

These functions remove all attributes / child nodes in bulk which is faster than removing them one at a time.

Migrate from licenseUrl to license since licenseUrl is deprecated and
fix grammar for VS version list.

Update changelog and regenerate HTML documentation.

According to XML spec, > sometimes needs to be escaped in PCDATA (when
it occurs as a ]]> pattern), but it doesn't need to be escaped in
attribute values.

Contributes to #272.

When using double quotes for attributes, we don't need to escape '; when
using single quotes, we don't need to escape ".

This changes behavior to match 1.9 by default (where we don't escape ').

Contributes to #272.

Note: this chang also updates PUGIXML_VERSION macro to allow for
double-digit minor versions; this preserves the continuity of versions
so PUGIXML_VERSION >= 190 will still work.

Create visual studio projects that are vs2019 compliant.
* nuget_build.ps1 :
Introduce a new argument that will define how we implement the nuget
build. For now we accept 201{9,7.5.3} as possible argument values.

* pugixml_vs2019{,_static}.vcxproj :
Add two visual studio projects that build pugi with the latest SDK and
build tools

* appveyor.yml
- Add Visual Studio 2019 to build targets
- Add Visual Studio 201{9,3,5} to build_scripts. And call
  nuget_build.ps1 with a new argument.
- Add Visual Studio 2019 to the test_scripts.

- Typos were found by codespell v1.15.0

Self-assignment generates a Wself-assign-overloaded warning.

appending the suffix to the build product need not be a function of
whether CMAKE_CONFIGURATION_TYPES is set.  for example, having two ninja
build trees---one for debug and another for release---is a fine use-case
for USE_POSTFIX.