progs/glsl: Include local headers before installed headers during compilation.

Fixes compilation errors on platforms with insufficient older installed
GL headers.

progs/glsl/Makefile

@@ -15,7 +15,7 @@ LIBS = -L$(TOP)/$(LIB_DIR) -l$(GLUT_LIB) -l$(GLEW_LIB) -l$(GLU_LIB) \
 
 # using : to avoid APP_CC pointing to CC loop
 CC := $(APP_CC)
-CFLAGS += -I$(INCDIR)
+CFLAGS := -I$(INCDIR) $(CFLAGS)
 LDLIBS = $(LIBS)
 
 PROG_SOURCES = \