build: Use pkg-config to find FreeType

build: Use pkg-config to find FreeType

pkg-config is apparently preferred over freetype-config these days (see
the BUGS section of freetype-config(1)).  pkg-config support was added
to FreeType in version 2.1.5, which was released in 2003, so it should
comfortably be available everywhere by now.

We no longer need to explicitly substitute FREETYPE_CFLAGS and
FREETYPE_LIBS, since PKG_CHECK_MODULES does that automatically.

Fixes Debian bug #887721.

Reported-by: Hugh McMaster <hugh.mcmaster@outlook.com>
Signed-off-by: Colin Watson <cjwatson@ubuntu.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>