travis-ci: Be more strict on build checks

After eliminating all warnings, we can disallow them for future
builds. This is done by passing an additional "-Werror" flag
from the Travis build step. Introducing new warnings will cause
a (CI) build failure from now on.
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>

.travis.yml

@@ -30,10 +30,10 @@ before_install:
       brew install libusb;
     fi
 
-# build by simply using the Makefile
+# build using the Makefile, treat all warnings as errors
 script:
-  - make
-  - make misc
+  - make EXTRA_CFLAGS=-Werror
+  - make misc EXTRA_CFLAGS=-Werror
 
 # turn off email notifications
 notifications:

Makefile

@@ -17,7 +17,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 CC ?= gcc
-CFLAGS = -g -O0 -Wall -Wextra
+CFLAGS = -g -O0 -Wall -Wextra $(EXTRA_CFLAGS)
 CFLAGS += -std=c99 $(DEFINES)
 CFLAGS += -Iinclude/