Fix check for whether we need -lm. Restore Ubuntu build.

sc2/build/unix/build.config

@@ -45,11 +45,19 @@ uqm_requirements()
 	define_have_symbol strcasecmp
 	define_have_symbol stricmp
 
+	# Check to see whether math functions are available for free.
+	define_have_symbol acos
+
 	# Require either strcasecmp or stricmp.
 	if not have_symbol strcasecmp && not have_symbol stricmp; then
 		echo "Fatal: Your system defines neither strcasecmp() nor stricmp()."
 		exit 1
 	fi
+
+	# If we don't have math for free, add -lm to LDFLAGS.
+	if not have_symbol acos; then
+		LDFLAGS="$LDFLAGS -lm"
+	fi
 	
 	# Add defines for HAVE_ISWGRAPH, HAVE_WCHAR_T, and HAVE_WINT_T
 	define_have_symbol iswgraph

sc2/build/unix/config_proginfo_build

@@ -143,7 +143,6 @@ case "$HOST_SYSTEM" in
 		BUILDTOOL_LINK_DEPEND='gcce'
 		;;
 	*)
-		EXTRA_PLATFORM_GCC_FLAGS_LINK='-lm'
 		useGccBuildTools
 		;;
 esac