Don't link to libGL at compile time

Get all OpenGL functions using SDL_GL_GetProcAddress(). This makes it
easier to cross-arch compile on Linux and add support for OpenGL ES
in the future.

Users still have to supply their own libSDL2 for cross-arch compiling
on Linux. But now the user does not have to re-install libgl1-mesa-dev
package for i386 or amd64 on Debian when switching between compiling
ioquake3 for x86 and x86_64.

Makefile

@@ -381,7 +381,7 @@ ifneq (,$(findstring "$(PLATFORM)", "linux" "gnu_kfreebsd" "kfreebsd-gnu" "gnu")
   AUTOUPDATER_LIBS += -ldl
 
   CLIENT_LIBS=$(SDL_LIBS)
-  RENDERER_LIBS = $(SDL_LIBS) -lGL
+  RENDERER_LIBS = $(SDL_LIBS)
 
   ifeq ($(USE_OPENAL),1)
     ifneq ($(USE_OPENAL_DLOPEN),1)
@@ -613,7 +613,7 @@ ifdef MINGW
     CLIENT_LDFLAGS += -mwindows
   endif
   CLIENT_LIBS = -lgdi32 -lole32
-  RENDERER_LIBS = -lgdi32 -lole32 -lopengl32 -static-libgcc
+  RENDERER_LIBS = -lgdi32 -lole32 -static-libgcc
 
   ifeq ($(USE_FREETYPE),1)
     FREETYPE_CFLAGS = -Ifreetype2
@@ -699,7 +699,7 @@ ifeq ($(PLATFORM),freebsd)
   CLIENT_LIBS =
 
   CLIENT_LIBS += $(SDL_LIBS)
-  RENDERER_LIBS = $(SDL_LIBS) -lGL
+  RENDERER_LIBS = $(SDL_LIBS)
 
   # optional features/libraries
   ifeq ($(USE_OPENAL),1)
@@ -790,7 +790,7 @@ ifeq ($(PLATFORM),openbsd)
   CLIENT_LIBS =
 
   CLIENT_LIBS += $(SDL_LIBS)
-  RENDERER_LIBS = $(SDL_LIBS) -lGL
+  RENDERER_LIBS = $(SDL_LIBS)
 
   ifeq ($(USE_OPENAL),1)
     ifneq ($(USE_OPENAL_DLOPEN),1)
@@ -853,7 +853,7 @@ ifeq ($(PLATFORM),irix64)
   # FIXME: The X libraries probably aren't necessary?
   CLIENT_LIBS=-L/usr/X11/$(LIB) $(SDL_LIBS) \
     -lX11 -lXext -lm
-  RENDERER_LIBS = $(SDL_LIBS) -lGL
+  RENDERER_LIBS = $(SDL_LIBS)
 
 else # ifeq IRIX
 
@@ -909,7 +909,7 @@ ifeq ($(PLATFORM),sunos)
   BOTCFLAGS=-O0
 
   CLIENT_LIBS +=$(SDL_LIBS) -lX11 -lXext -liconv -lm
-  RENDERER_LIBS = $(SDL_LIBS) -lGL
+  RENDERER_LIBS = $(SDL_LIBS)
 
 else # ifeq sunos