configure: Support preprocessor macros as header names

New versions of FreeType have moved the location of their API
header(s) and hide the location behind a macro.

Since the location changes between versions and no other way
to know the location exists, this workaround becomes necessary.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
(cherry picked from commit 52ccc4a0ece88030e67254418317d72089a0ecc8)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
drawtext: Drop pointless header

It should be forward compatible with newer freetype.

(cherry picked from commit d68dc3c9446e38b4d686cc0f55433c9e8d7c128b)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
configure: Update freetype check to follow upstream

The freetype tutorial suggests to use #include FT_FREETYPE_H.

Bug-Id: 616
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

(cherry picked from commit e61b8fa5605b16a02a2a0ea75afbfc31d7832bba)