Makefile: remove unneeded -I/usr/include from flags

gcc warns about an include directive that shadows the system include
directory at high verbosity levels. Let's filter it out (pkgconfig
implements that) to reduce noise and also to make the command line
shorter.

Author: keszybz

Makefile

@@ -3,6 +3,7 @@ SED = sed
 SPHINX_BUILD = sphinx-build
 ETAGS = etags
 INCLUDE_DIR := $(shell pkg-config --variable=includedir libsystemd)
+INCLUDE_FLAGS := $(shell pkg-config --cflags libsystemd)
 VERSION := $(shell $(PYTHON) setup.py --version)
 TESTFLAGS = -v
 
@@ -31,7 +32,7 @@ update-constants: $(INCLUDE_DIR)/systemd/sd-messages.h
 	mv docs/id128.rst{.tmp,}
 
 build:
-	$(PYTHON) setup.py build_ext $(if $(INCLUDE_DIR),-I$(INCLUDE_DIR))
+	$(PYTHON) setup.py build_ext $(INCLUDE_FLAGS)
 	$(PYTHON) setup.py build
 
 install: