Gcc 6 reports a warning on narrowing conversions. Turn this warning
off, and don't build with -Werror as it fails for some other reasons.

Upstream is not interested in this patch because they work on a
reimplementation of the game engine. Maintaining this one is not in
their top priorities.
---
--- a/src/texture.hpp
+++ b/src/texture.hpp
@@ -58,7 +58,7 @@
 	unsigned int get_id() const;
 	static void set_current_texture(unsigned int id);
 	void set_as_current_texture() const;
-	bool valid() const { return id_; }
+	bool valid() const { return (bool) id_; }
 
 	static texture get(const std::string& str, int options=0);
 	static texture get(const std::string& str, const std::string& algorithm);
--- a/Makefile
+++ b/Makefile
@@ -34,14 +34,14 @@ BASE_CXXFLAGS += -O2
 endif
 
 # Initial compiler options, used before CXXFLAGS and CPPFLAGS.
-BASE_CXXFLAGS += -g -fno-inline-functions -fthreadsafe-statics -Wnon-virtual-dtor -Werror -Wignored-qualifiers -Wformat -Wswitch
+BASE_CXXFLAGS += -g -fno-inline-functions -fthreadsafe-statics -Wnon-virtual-dtor -Wignored-qualifiers -Wformat -Wswitch -Wno-error=deprecated-declarations -Wno-error=narrowing
 
 # Compiler include options, used after CXXFLAGS and CPPFLAGS.
 INC := $(shell pkg-config --cflags x11 sdl glu glew SDL_image libpng zlib)
 
 # Linker library options.
 LIBS := $(shell pkg-config --libs x11 ) -lSDLmain \
-	$(shell pkg-config --libs sdl glu glew SDL_image libpng zlib) -lSDL_ttf -lSDL_mixer
+	$(shell pkg-config --libs sdl glu glew SDL_image libpng zlib) -lGL -lSDL_ttf -lSDL_mixer
 
 include Makefile.common
 
