pkg-config names can be encoded with a version requirement, such as
zlib>=1.2.3, thus we need to remove that from the variable.
with autotools we used HAVE_XXX and ENABLED_XXX to instruct such
optional library was present, then define that just for the user
target by adding that to its CFLAGS.
This allows us to remove some defines that matches the name, only
leave those that translate from original pkg-config name, such as
libsystemd->systemd.