lang/gnat12: Makefile maintenance

• Move gettext-runtime to NLS helper, including pkg-plist updates;
• Prefix all custom variable with an underscore
• Sort Makefile Blocks into correct order; this meant breaking down the one if conditional covering MULTILIB into multiple conditionals spread between each Block
• Remove ${INFOS}; this was probably meant to be ${INFO}. The Info pages are removed later in the post-stage target, so there is no point defining any.
• No need to define ${GNU_CONFIGURE_MANPREFIX}, the Ports framework inherits this from ${MANPREFIX}, which is also defined in the port Makefile
• Explicitly enable C++ language to GNU configure; GNAT requires a working C++ toolchain to build itself
• Move to bsd.port.options.mk/bsd.port.mk Makefile inclusion pair
• Prefer ${DISTVERION} over ${PORTVERSION}
• Remove @ from some Makefile ${COMMANDS} to prefer verbose build output

PR: 274619
Approved by: /me