MACOS: Drop PowerPC static builds workarounds, and suggest dynamic plugins

-mlongcall -Os would work until ScummVM 2.2.0, but now the engines are
too big to fit in a single static binary on Mac PowerPC.

(On ELF platforms, some flags like -Wl,--relax or -Wl,--gc-sections
can be used as well, but macOS uses Mach-O, and its old linker doesn't
have as many workarounds.  -Wl,-dead_strip helped a bit but that's not
enough and you start hitting ld internal errors.)

So, recommend dynamic plugins on Mac PPC, now, and drop -Os -mlongcall
since working around the linker limits is not possible anymore.  Using
compiler defaults probably means less bugs when using such an obscure
toolchain, anyway.

(Making a static release it still possible, but only with a very small
set of engines.)

configure

@@ -2655,17 +2655,11 @@ case $_host_os in
 		exit 1
 		;;
 	darwin*)
-		# Pass -mlongcall to gcc so that it emits long calls
-		# which will allow for calls larger than 32MB. The linker
-		# will discard the calls if they are not needed, but we
-		# need to ensure the compiler emits them in the first place.
-		# Also the executable has grown to a size where using -Os is necessary to avoid a
-		# 'virtual memory exhausted' error when running the executable.
 		case $_host_cpu in
 		powerpc*)
-			append_var CFLAGS "-mlongcall"
-			append_var CXXFLAGS "-mlongcall"
-			_optimization_level=-Os
+			if test "$_dynamic_modules" = no ; then
+				echo "WARNING: Building static engines will probably fail at link time on Mac PowerPC"
+			fi
 			;;
 		esac