Makefile: Factor out flags and respect user-defined CFLAGS/LDFLAGS

Makefile

@@ -1,18 +1,21 @@
-GIMPARGS = $(shell gimptool-2.0 --cflags --libs)
+GIMP_CFLAGS = $(shell gimptool-2.0 --cflags)
+GIMP_LIBS = $(shell gimptool-2.0 --libs)
+CFLAGS += $(GIMP_CFLAGS) -Wall -Wno-unused-variable -Wno-pointer-sign -DGIMP_DISABLE_DEPRECATED
+LDFLAGS += $(GIMP_LIBS) -lm
 SYSTEM_INSTALL_DIR = $(shell gimptool-2.0 --dry-run --install-admin-bin ./bin/bimp | sed 's/cp \S* \(\S*\)/\1/'|tr -d [\'])
 USER_INSTALL_DIR = $(shell gimptool-2.0 --dry-run --install-bin ./bin/bimp | sed 's/cp \S* \(\S*\)/\1/'|tr -d [\'])
 
 make: 
 	which gimptool-2.0 && \
-	gcc -o ./bin/bimp -Wall -O2 -Wno-unused-variable -Wno-pointer-sign src/*.c src/manipulation-gui/*.c src/images/*.c $(GIMPARGS) -lm -DGIMP_DISABLE_DEPRECATED
+	gcc -o ./bin/bimp -O2 $(CFLAGS) src/*.c src/manipulation-gui/*.c src/images/*.c $(LDFLAGS)
 
 makewin-debug: 
 	which gimptool-2.0 && \
-	gcc -o ./bin/win32/bimp -Wall -Wno-unused-variable -Wno-pointer-sign src/*.c src/manipulation-gui/*.c src/images/*.c $(GIMPARGS) -lm -DGIMP_DISABLE_DEPRECATED
+	gcc -o ./bin/win32/bimp $(CFLAGS) src/*.c src/manipulation-gui/*.c src/images/*.c $(LDFLAGS)
 
 makewin: 
 	which gimptool-2.0 && \
-	gcc -mwindows -o ./bin/win32/bimp -O2 -Wall -Wno-unused-variable -Wno-pointer-sign src/*.c src/manipulation-gui/*.c src/images/*.c $(GIMPARGS) -lm -DGIMP_DISABLE_DEPRECATED
+	gcc -mwindows -o ./bin/win32/bimp -O2 $(CFLAGS) src/*.c src/manipulation-gui/*.c src/images/*.c $(LDFLAGS)
 
 install: 
 	mkdir -p "$(USER_INSTALL_DIR)"