Prior to this commit, the include path for some libraries found through
pkg-config when cross compiling is not correct:
$ grep include CMakeCache.txt
blas_INCLUDEDIR:INTERNAL=/usr/include/x86_64-linux-gnu
...
This is because we did not tell pkg-config to search in the right path.
Since for cross-compilation, we install packages in parallel in the
system, override the primary directory where the .pc files located, in
the cross-compilation toolchain file for cmake.
Closes: #746
Robert Schmidt