ZynAddSubFX/FLTK: Yet another fix for finding freetype

Revert changes of commit 17babf6abc3d5de09bbbb4e5b72248a530c47728 as commit 26665dde55cfa589f166998ea587db70cd516b68 was complete indeed regarding the FIND_PACKAGE statement.
Add REQUIRED statement to FIND_PACKAGE(Freetype)
Use FREETYPE_INCLUDE_DIRS instead of FREETYPE_INCLUDE_DIR_freetype2 or FREETYPE_INCLUDE_DIR_ft2build
Merge pull request #36 from Lukas-W/stable-0.4

ZynAddSubFX/FLTK: Yet another fix for finding freetype