build: Combine $(CFLAGS) and $(STRICT_CFLAGS).

build: Combine $(CFLAGS) and $(STRICT_CFLAGS).

We used to compile the parsers generated by gengetopt with less strict
flags to avoid warnings in generated files. With lopsub this is no
longer necessary, so one set of flags suffices.

Moreover, we currently initialize $(STRICT_CFLAGS) to $(CFLAGS) (from
the environment), then append our own flags. This is unfortunate
because the user defined settings in $(CFLAGS) are overwritten.

This patch changes the Makefile to only set STRICT_CFLAGS. We now
pass $(STRICT_CFLAGS) $(CFLAGS) to the compiler, allowing the user
to override (parts of) $(STRICT_CFLAGS) by adding -Wno-foo to CFLAGS.

We use the opportunity to regroup the various options a little.