generate_visualc_files.pl has a list of directories that it pulls
headers from, so it knows what directories to put on the include path.
Make it inject the include path into the output files, rather than
hard-coding the include paths in template files.
A similar change (but with different code) was made in Mbed TLS in
commit b78cf2b261
"Adjust visual studio file generation to always use the crypto submodule".
No semantic change: this commit does not change the generated files.
This being the first 3rdparty-contribution, we may want to consider the
structure of the project file generation scripts. Perhaps add small,
constribution-specific scripts to each directory in 3rdparty instead of adding
all constraints to generate_visualc_files.pl?
Adds interim directories to the Visual Studio project files to avoid warning
MSB8028 in Visual Studio 2015, where shared directories of intermediate files
between project files generate the warning.
