meson advice on CFLAGS et al.

Hi!

The meson documentation says to _not_ use CFLAGS, CXXFLAGS, LDFLAGS
(and some more), but to use meson command line arguments instead.

https://mesonbuild.com/Reference-tables.html#language-arguments-parameter-name

Using this did actually fix passing the linker flags for
graphics/geeqie, so I wonder if we should set c_args, c_link_args,
cpp_args, and cpp_link_args in meson/build.mk by default.

I don't think we can easily remove the settings for CFLAGS, CXXFLAGS,
and LDFLAGS from the pkgsrc environment there though.
 Thomas