[Clacc] Document working from build directory

base language.

Clacc's compiler is the `clang` executable in the `bin` subdirectory

of the install directory.  It's also possible to work from the build

directory, but additional effort is usually then required to help

`clang` find its own libraries and header files.

Here's a simple example of using Clacc's `clang`, where

of the install directory.  Here's a simple example of using it, where

`$CLACC_INSTALL_DIR` is `$LLVM_GIT_DIR/install` when following the

build procedure above:

  return 0;

}

```

## Usage from a Build Directory

Most Clacc users should work from the install directory, as described

in the previous section.  However, if you plan to modify Clacc, it

might be easier to work from the build directory instead.  Doing so

requires setting many environment variables and command-line options

to ensure `clang` finds its own libraries and header files regardless

of any like-named files that happen to be installed in system

directories.  Not setting those can produce unexpected behavior at

compile time or run time.  This complexity is inherited from LLVM

upstream and is not unique to Clacc.

For example, to compile and run for an NVIDIA GPU, where

`$CLACC_BUILD_DIR` is `$LLVM_GIT_DIR/build` when following the build

procedure above:

```

$ export PATH=$CLACC_BUILD_DIR/bin:$PATH

$ export LIBRARY_PATH=$CLACC_BUILD_DIR/runtimes/runtimes-bins/openmp/libomptarget:$LIBRARY_PATH

$ export LD_LIBRARY_PATH=$CLACC_BUILD_DIR/runtimes/runtimes-bins/openmp/runtime/src:$CLACC_BUILD_DIR/runtimes/runtimes-bins/openmp/libomptarget:$CLACC_BUILD_DIR/runtimes/runtimes-bins/openmp/libacc2omp/src:$LD_LIBRARY_PATH

$ clang -fopenacc -fopenmp-targets=nvptx64-nvidia-cuda \

  -L $CLACC_BUILD_DIR/runtimes/runtimes-bins/openmp/runtime/src \

  -L $CLACC_BUILD_DIR/runtimes/runtimes-bins/openmp/libomptarget \

  -L $CLACC_BUILD_DIR/runtimes/runtimes-bins/openmp/libacc2omp/src \

  -isystem $CLACC_BUILD_DIR/runtimes/runtimes-bins/openmp/runtime/src \

  -isystem $CLACC_BUILD_DIR/runtimes/runtimes-bins/openmp/libacc2omp/src \

  test.c

$ ./a.out

```

## Compiler Options

The most relevant `clang` command-line options are as follows: