linux: Split off dependencies' CPPFLAGS
linux: Split off dependencies' CPPFLAGS

Use MAINT_CPPFLAGS only for stuff used directly by spectrwm, and
move all the stuff needed by dependencies to the newly-defined
BIN_CPPFLAGS and LIB_CPPFLAGS.

conformal/spectrwm,conformal/spectrwm,conformal/spectrwm