Our FLTK patches modified FLTK's autotools-based build system so that HAVE_XFIXES...

Our FLTK patches modified FLTK's autotools-based build system so that HAVE_XFIXES and HAVE_XCURSOR were defined in FLTK's config.h, but those changes never made it into the CMake-based build system used by the in-tree version of FLTK.  Further, our build system was allowing silent failures whenever Xft, Xinerama, Xcursor, or Xfixes were not present on the build system.  Now, the lack of these libraries is treated as a fatal error, since these libraries are critical for TigerVNC functionality.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4834 3789f03b-4d11-0410-bbf8-ca57d06f2519