This means: this strategy *would* work if someone didn't have pkg-config
*and* set the correct CFLAGS/CXXFLAGS/LDFLAGS for their system. Whether
this is useful I can't tell, but it *does* work.

We could do something similar for the detection of xft/freetype, i.e.
not abort the configure job but continue anyway if pkg-config is not
available. Again, the user would have to provide the correct flags in
CFLAGS/CXXFLAGS/LDFLAGS instead.

That said, configure would still abort if it didn't *find* the libs, but
you wouldn't have to install pkg-config if you can set the
CFLAGS/CXXFLAGS/LDFLAGS properly for your system.

BTW: we're also using pkg-config for the detection of Cairo libs. We
could take a similar approach for this as well.