x11/pcdm: fix build with GCC-based architectures

This port needs USES=compiler:c++11-lang to build on GCC architectures.

Additionally (by fernape):

• Reorder USE in the Makefile
• Add widgets as a USE_QT member
• Add USE_GL and USE_XORG (suggested by the framework)
• Fix wrong %%ETCDIR%% substitution in pkg-plist

PR: 237117
Approved by: linimon (mentor)