pbd/i18n.h MUST NEVER be included from header files and always be
the last include. This is because `_` is declared other headers
notably boost and some apple headers.
leading to issues like
../libs/pbd/gettext.h:58:27: error: expected unqualified-id before ‘const’
58 | # define gettext(Msgid) ((const char *) (Msgid))
Previously this was inherited via PBD.
On MacOS/X, this adds
"-undefined dynamic_lookup -flat_namespace"
and various "-framework .." options to linkflags
Without this flag, .dylibs fail to link usually because
of missing `-lintl` (Undefined symbols: "_libintl_dgettext")
On other systems this is a NO-OP:
CFLAGS_OSX, CXXFLAGS_OSX and LINKFLAGS_OSX
are only set on the darwin platform.
