make: improve cross compilation support

resolves #14.

Signed-off-by: Vanya A. Sergeev <v@sergeev.io>

Makefile

@@ -32,7 +32,7 @@ clean:
 ###########################################################################
 
 tests/%: tests/%.c $(LIB)
-	$(CC) $(CFLAGS) $(LDFLAGS) $< $(LIB) -o $@ -lpthread
+	$(CROSS)$(CC) $(CFLAGS) $(LDFLAGS) $< $(LIB) -o $@ -lpthread
 
 ###########################################################################
 
@@ -42,8 +42,8 @@ $(OBJDIR):
 	mkdir $(OBJDIR)
 
 $(LIB): $(OBJECTS)
-	ar rcs $(LIB) $(OBJECTS)
+	$(CROSS)$(AR) rcs $(LIB) $(OBJECTS)
 
 $(OBJDIR)/%.o: $(SRCDIR)/%.c
-	$(CC) $(CFLAGS) $(LDFLAGS) -c $< -o $@
+	$(CROSS)$(CC) $(CFLAGS) $(LDFLAGS) -c $< -o $@
 

README.md

@@ -294,18 +294,18 @@ $
 
 ### Cross-compilation
 
-Set the `CC` environment variable with the cross-compiler when calling make:
+Set the `CROSS` and `CC` environment variables with the cross-compiler prefix and compiler, respectively, when calling make:
 
 ``` console
-$ CC=arm-linux-gcc make clean all tests
+$ CROSS=arm-linux- CC=gcc make clean all tests
 rm -rf periphery.a obj tests/test_serial tests/test_i2c tests/test_mmio tests/test_spi tests/test_gpio
 mkdir obj
 arm-linux-gcc -Wall -Wextra -Wno-unused-parameter -Wno-pointer-to-int-cast  -fPIC  -c src/gpio.c -o obj/gpio.o
 arm-linux-gcc -Wall -Wextra -Wno-unused-parameter -Wno-pointer-to-int-cast  -fPIC  -c src/spi.c -o obj/spi.o
 arm-linux-gcc -Wall -Wextra -Wno-unused-parameter -Wno-pointer-to-int-cast  -fPIC  -c src/i2c.c -o obj/i2c.o
 arm-linux-gcc -Wall -Wextra -Wno-unused-parameter -Wno-pointer-to-int-cast  -fPIC  -c src/mmio.c -o obj/mmio.o
 arm-linux-gcc -Wall -Wextra -Wno-unused-parameter -Wno-pointer-to-int-cast  -fPIC  -c src/serial.c -o obj/serial.o
-ar rcs periphery.a obj/gpio.o obj/spi.o obj/i2c.o obj/mmio.o obj/serial.o
+arm-linux-ar rcs periphery.a obj/gpio.o obj/spi.o obj/i2c.o obj/mmio.o obj/serial.o
 arm-linux-gcc -Wall -Wextra -Wno-unused-parameter -Wno-pointer-to-int-cast  -fPIC  tests/test_serial.c periphery.a -o tests/test_serial
 arm-linux-gcc -Wall -Wextra -Wno-unused-parameter -Wno-pointer-to-int-cast  -fPIC  tests/test_i2c.c periphery.a -o tests/test_i2c
 arm-linux-gcc -Wall -Wextra -Wno-unused-parameter -Wno-pointer-to-int-cast  -fPIC  tests/test_mmio.c periphery.a -o tests/test_mmio