sysdeps: Search C include files in multiarch subdirectories

Since version 3.9.7-2 of the libtiff4-dev Debian package, the tiff.h
header file has been moved to /usr/include/x86_64-linux-gnu/tiff.h and
jhbuild was unable to detect its availability when checking for system
dependencies.

Adding "/usr/include/`gcc -print-multiarch`" to the search paths fixes
the problem.

https://bugzilla.gnome.org/show_bug.cgi?id=707561