Using clang-sys for header paths breaks any non-default-target builds

[vvuk]

This breaks msvc (or really any non-default-target bindings generation):

When llvm/clang is built for the gnu target by default (needed for msvc bindings generation, because it's easier to use those default include paths and let the msvc target get them from $INCLUDE), clang behaves properly:

$ vladimir@glacier[5307]$ clang -E -x c++ - -v
Target: x86_64-w64-windows-gnu
[...]
#include "..." search starts here:
#include <...> search starts here:
 W:\msys64\mingw64\include\c++\6.1.0
 W:\msys64\mingw64\include\c++\6.1.0\x86_64-w64-mingw32
 W:\msys64\mingw64\include\c++\6.1.0\backward
 C:\proj\llvm\build-mingw64\bin\..\lib\clang\3.9.0\include
 W:\msys64\mingw64\lib\gcc\x86_64-w64-mingw32\6.1.0\include
 W:\msys64\mingw64\lib\gcc\x86_64-w64-mingw32\6.1.0\include-fixed
 W:\msys64\mingw64\x86_64-w64-mingw32\include
 W:\msys64\mingw64\include
End of search list.

$ vladimir@glacier[5307]$ clang --target=x86_64-pc-win32 -E -x c++ - -v
Target: x86_64-pc-windows-msvc
[...]
#include "..." search starts here:
#include <...> search starts here:
 C:\proj\llvm\build-mingw64\bin\..\lib\clang\3.9.0\include
 C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include
 C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\atlmfc\include
 C:\Program Files (x86)\Windows Kits\10\include\10.0.10586.0\ucrt
 C:\Program Files (x86)\Windows Kits\10\include\10.0.10586.0\shared
 C:\Program Files (x86)\Windows Kits\10\include\10.0.10586.0\um
 C:\Program Files (x86)\Windows Kits\10\include\10.0.10586.0\winrt
End of search list.

But when rust-bindgen is passed --target=x86_64-pc-win32, it gets:

$ ./etc/bindings.sh msvc14
...
Target: x86_64-pc-windows-msvc
#include "..." search starts here:
#include <...> search starts here:
 ../target/debug/build/mozjs_sys-4ab490f4d99d7124/out/dist/include
 W:\msys64\mingw64\include\c++\6.1.0
 W:\msys64\mingw64\include\c++\6.1.0\x86_64-w64-mingw32
 W:\msys64\mingw64\include\c++\6.1.0\backward
 C:\proj\llvm\build-mingw64\bin\..\lib\clang\3.9.0\include
 W:\msys64\mingw64\lib\gcc\x86_64-w64-mingw32\6.1.0\include
 W:\msys64\mingw64\lib\gcc\x86_64-w64-mingw32\6.1.0\include-fixed
 W:\msys64\mingw64\x86_64-w64-mingw32\include
 W:\msys64\mingw64\include
 C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include
 C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\atlmfc\include
 C:\Program Files (x86)\Windows Kits\10\include\10.0.10586.0\ucrt
 C:\Program Files (x86)\Windows Kits\10\include\10.0.10586.0\shared
 C:\Program Files (x86)\Windows Kits\10\include\10.0.10586.0\um
 C:\Program Files (x86)\Windows Kits\10\include\10.0.10586.0\winrt
End of search list.

which then totally breaks the generation.

I'm going to hack around this by skipping the clang-sys isystem stuff (why is it needed?) if --target is specified.

[emilio]

This was fixed in #54, thanks vlad :)