Add MACOS_FRAMEWORK to static link flags

Add MACOS_FRAMEWORK to static link flags

When building statically on macOS, the CoreFoundation and Security
frameworks required by krb5 are not propagated to consumers via
pkg-config or the Makefile link variables.  This causes linker failures
when an application links against the static krb5 libraries without
separately specifying the framework flags.

Add @MACOS_FRAMEWORK@ to Libs.private in mit-krb5.pc.in, so pkg-config
--static exposes it, and to KRB5_BASE_LIBS in config/pre.in, so the
build system includes it when linking executables against the static
libraries.