misc/mc{,-nox11}: slightly improve the ports' Makefiles (+)

• Do not globally pollute -I/-L with ${LOCALBASE} when handling the NLS option and pass it as the --with-libintl-prefix instead
• Drop no longer supported --with-slang-includes configure switch, correct paths are obtained via pkgconf(1) these days
• Set PKGNAMESUFFIX together with CONFLICTS_INSTALL in the master so they are always coherent (previously, when the main port is built with X11 option unset it would conflict with itself)