Fix build system to honour all user variables.

--- a/src/Makefile
+++ b/src/Makefile
@@ -1,4 +1,4 @@
 all:
-	g++ main.cpp -Wall `/usr/bin/sdl-config --libs --cflags` -lSDL_ttf  -o ../TuxDash
+	$(CXX) main.cpp $(LDFLAGS) $(CXXFLAGS) $(CPPFLAGS) `/usr/bin/sdl-config --libs --cflags` -lSDL_ttf  -o ../tuxdash
 static:
-	g++ -static main.cpp -Wall `/usr/bin/sdl-config --cflags --static-libs` -lSDL_ttf -lfreetype -lz -o ../TuxDash
+	$(CXX) -static main.cpp $(LDFLAGS) $(CXXFLAGS) $(CPPFLAGS) `/usr/bin/sdl-config --cflags --static-libs` -lSDL_ttf -lfreetype -lz -o ../tuxdash