pkg-config: only prepend sysroot to includedir / libdir

Only append sysroot to includedir / libdir, and not to other variables,
when a variable is requested (--variable=<name>), similar to how it
is done for the -I / -L flags in cflags/ldflags.

The problem is that pkg-config doesn't have any explicit cross compilation
support, so there's some confusion about what are host paths and what are
target ones.  Includedir / libdir are paths for the (cross) compiler, so
those should normally have sysroot prepended, but other variables might
refer to target paths instead.

This fixes the installation of the xfonts-* packages, which uses
pkg-config --variable=fontrootdir fontutil to figure out the target
installation location for fonts.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>