libgd: fix building with freetype support

libgd: fix building with freetype support

Freetype support is required for php7-mod-gd as of commit 0f10c8c8,
which causes the PHP7 package to build using this external libgd library.
This commit adds FREETYPE_INCLUDE_DIRS to the definition of CMAKE_OPTIONS.
Without this, libgd does not build freetype support as shown by
this message:

....
Build libgd:
-- Could NOT find Freetype (missing:  FREETYPE_INCLUDE_DIRS)
-- Configuring done
-- Generating done
....

Signed-off-by: W. Michael Petullo <redacted>