Fix libtiff version in pkg-config
Without this, the installed libtiff-4.pc just says "Version: ". Packages that depend upon libtiff with a version constraint (eg: ">=4.0.0") fail to build because they cannot determine if the installed libtiff satisfies the version constraint.
This fixes a regression introduced in a28689d5 which switched the VERSION variables to the implicit ones that the cmake "project" command defines.
gegl (and thus the GIMP) cannot build against libtiff 4.3.0. This fix will allow them to build against the next libtiff release.