pkgconf: fix incorrect variable-directory sysroot prefixing

According to the pkg-config documents ("specification") when
sysroot-prefixing is enabled via PKG_CONFIG_SYSROOT_DIR this should only
be applied to -I/-L directories (includes, library directories), and not
to all of the other variables that can specify a directory.

However xorg uses mapdir/sdkdir in a similar fashion in what could be
considered an abuse of the spec, hence needs to be prefixed as well.
And what's more, it also uses includedir in a nonstandard fashion just
requesting the value via 'pkg-config --variable=includedir libfoo' which
doesn't pass the standard prefixing rules for the --cflags and --libs
invocation.

This patch makes pkgconf behave in the pkg-config specified way with the
added exception for the includedir, libdir, mapdir and sdkdir variables
which are prefixed.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>