gnupg2: refresh GCC options in Makefile

gnupg2: refresh GCC options in Makefile

this Makefile still used `CONFIG_GCC_USE_VERSION_*` to select various
compilation options, for GCC versions that are antiquated

convert to parsing the major from the `CONFIG_GCC_VERSION` which will
always exist and can also be used with range logic

intent seemed to be:
* `-DEXTERN_UNLESS_MAIN_MODULE=static` for "=10" (and newer, probably)
* no additional options for "not =10" (or older, probably)

GCC 11 or 12 would likely revert to the default (not =10) option,
because 10 was the newest at the time, and 11 and 12 are "not 10"

Signed-off-by: Tony Butler <spudz76@gmail.com>