travis: Actually build with clang if we want to

.travis.yml

@@ -26,8 +26,8 @@ env:
 script:
   - docker pull cdecker/lightning-ci:${ARCH}bit | tee
   - env | grep -E '^[A-Z_]+\=' | tee /tmp/envlist
-  - docker run --rm=true --env-file=/tmp/envlist -v "${TRAVIS_BUILD_DIR}":/build -t cdecker/lightning-ci:${ARCH}bit ./configure
-  - $SOURCE_CHECK_ONLY || docker run --rm=true --env-file=/tmp/envlist -v "${TRAVIS_BUILD_DIR}":/build -t cdecker/lightning-ci:${ARCH}bit make -j3 
+  - docker run --rm=true --env-file=/tmp/envlist -v "${TRAVIS_BUILD_DIR}":/build -t cdecker/lightning-ci:${ARCH}bit ./configure CC=${COMPILER:-gcc}
+  - $SOURCE_CHECK_ONLY || docker run --rm=true --env-file=/tmp/envlist -v "${TRAVIS_BUILD_DIR}":/build -t cdecker/lightning-ci:${ARCH}bit make -j3
   - $SOURCE_CHECK_ONLY || docker run --rm=true --env-file=/tmp/envlist -v "${TRAVIS_BUILD_DIR}":/build -t cdecker/lightning-ci:${ARCH}bit make check
   - (! $SOURCE_CHECK_ONLY) || git clone https://github.com/lightningnetwork/lightning-rfc.git
   - (! $SOURCE_CHECK_ONLY) || docker run --rm=true -v "${TRAVIS_BUILD_DIR}":/build -t cdecker/lightning-ci:${ARCH}bit make clean