travis-ci: Extend build matrix, disable email notifications

We'll be testing both gcc and clang builds on Linux. As for now,
Travis CI only supports clang for OSX - so exclude gcc currently.
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>

.travis.yml

@@ -6,6 +6,16 @@ language: c
 os:
   - linux
   - osx
+compiler:
+  - gcc
+  - clang
+
+# OSX uses Apple's flavor of clang anyway, so there's no point in using "gcc".
+# This excludes the "gcc" compiler from the build matrix for OSX:
+matrix:
+  exclude:
+    - os: osx
+      compiler: gcc
 
 # take care of the libusb dependency for Linux
 addons:
@@ -24,3 +34,7 @@ before_install:
 script:
   - make
   - make misc
+
+# turn off email notifications
+notifications:
+  - email: false

Makefile

@@ -16,7 +16,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-CC = gcc
+CC ?= gcc
 CFLAGS = -g -O0 -Wall -Wextra
 CFLAGS += -std=c99 $(DEFINES)
 CFLAGS += -Iinclude/