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_GUI is conditional on GUI.

While here, tweak Makefile statement ordering to reduce portlint warnings.

PR: 234651
Approved by: makc (maintainer)
MFH: 2019Q1