Append solid suffix to non-transparent variant

This allows distributions to easily pacakge both without conflicts.
This will also help with Flatpak being able to match the host theme.

common/Makefile.am

@@ -1,6 +1,12 @@
+if !ENABLE_TRANSPARENCY
+themedir        = $(datadir)/themes/Arc-solid
+themedarkerdir  = $(datadir)/themes/Arc-Darker-solid
+themedarkdir    = $(datadir)/themes/Arc-Dark-solid
+else
 themedir        = $(datadir)/themes/Arc
 themedarkerdir  = $(datadir)/themes/Arc-Darker
 themedarkdir    = $(datadir)/themes/Arc-Dark
+endif
 
 ithemedir       = $(DESTDIR)$(themedir)
 ithemedarkerdir   = $(DESTDIR)$(themedarkerdir)