configure: Allow passing extra Objective C compiler flags

configure: Allow passing extra Objective C compiler flags

We can pass C/CPP/LD flags via CFLAGS/CXXFLAGS/LDFLAGS environment
variables, or via configure --extra-cflags / --extra-cxxflags /
--extra-ldflags options. Provide similar behavior for Objective C:
use existing flags from $OBJCFLAGS, or passed via --extra-objcflags.

Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Tested-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>