Revert automatic support for header-only mode since it creates problems with qmake.

Qmake treats all files that are #include-d as header files, even if the #include is
guarded by an #ifdef. It looks like the only solution that allows for transparent
header-only support based on preprocessor define involves moving the actual source
into a separate header file and including this file in pugixml.cpp.

Let's not do it yet.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@990 99668b35-9821-0410-8761-19e4c4f06640