linux/Makefile: Add support for cross compilation

0. VDR`s `Makefile` was taken as a template [1].
1. Separate variables for flags for `CC` and `CXX` were introduced. `-c` is now passed explicitly. Further flags for `CXX` could be added like in VDR’s `Makefile`.
2. Separate variable `INCLUDES` defined which is only used for the libraries(?).

[1] http://git.gekrumbel.de/vdr.git?p=vdr.git;a=blob;f=Makefile;hb=bd61fee1e9328aec470c319494cccb90e53302c2
--
Please note that this is not build tested.

linux/Makefile

@@ -9,22 +9,24 @@
 all: hidtest
 
 CC?=gcc
+CFLAGS?=-Wall -g
 CXX?=g++
+CXXFLAGS?=-Wall -g
 COBJS=hid-libusb.o
 CPPOBJS=../hidtest/hidtest.o
 OBJS=$(COBJS) $(CPPOBJS)
-CFLAGS+=-I../hidapi -Wall -g -c `pkg-config libusb-1.0 --cflags`
 LIBS=`pkg-config libusb-1.0 libudev --libs`
+INCLUDES?=-I../hidapi `pkg-config libusb-1.0 --cflags`
 
 
 hidtest: $(OBJS)
-	$(CXX) -Wall -g $(LDFLAGS) $^ $(LIBS) -o hidtest
+	$(CXX) $(CXXFLAGS) $(LDFLAGS) $^ $(LIBS) -o hidtest
 
 $(COBJS): %.o: %.c
-	$(CC) $(CFLAGS) $< -o $@
+	$(CC) $(CFLAGS) -c $(INCLUDES) $< -o $@
 
 $(CPPOBJS): %.o: %.cpp
-	$(CXX) $(CFLAGS) $< -o $@
+	$(CXX) $(CXXFLAGS) -c $(INCLUDES) $< -o $@
 
 clean:
 	rm -f $(OBJS) hidtest