linux: Reduce linking for libswmhack
linux: Reduce linking for libswmhack

The libswmhack library uses dlopen() and friends, so it needs
to link against libdl; on the other hand, it doesn't use nearly
as many library as spectrwm itself, so linking against them is
pointless.

Split MAINT_LDLIBS into BIN_LDLIBS and LIB_LDLIBS, and use them
for spectrwm and libswmhack respectively.

conformal/spectrwm,conformal/spectrwm,conformal/spectrwm