Make USES=gl conditional on the GUI option to unbreak build when GUI is
disabled. Error was:
scilab-5.5.2_15 need to specify gl component with USE_GL.
because USE_GL is conditional on GUI.
While here, tweak Makefile statement ordering to reduce portlint warnings.
Approved by: makc (maintainer)
Approved by: portmgr (blanket: build fix)