try clang on travis

Cherry-pick: a5327ba

.travis.yml

@@ -3,12 +3,18 @@ sudo: required
 dist: trusty
 compiler:
   - gcc
+  - clang
 os:
   - linux
 install:
   - sudo apt-get install libboost-dev
   - sudo apt-get install libprotobuf-dev protobuf-compiler libprotoc-dev
   - sudo apt-get install libboost-test-dev libboost-program-options-dev
-  - sudo apt-get install zlib1g-dev libc-ares-dev libcurl4-openssl-dev
+  - sudo apt-get install libc-ares-dev libcurl4-openssl-dev
+  - sudo apt-get install zlib1g-dev libgd-dev
+env:
+  - BUILD_TYPE=debug
+  - BUILD_TYPE=release
 script:
+  - if [ "$CXX" == "clang++" ]; then patch -p1 < clang.diff; fi
   - ./build.sh

clang.diff

@@ -1,19 +1,21 @@
 diff --git a/CMakeLists.txt b/CMakeLists.txt
-index a3066c5..9728303 100644
+index 81bd126..38c7387 100644
 --- a/CMakeLists.txt
 +++ b/CMakeLists.txt
-@@ -16,6 +16,7 @@
+@@ -18,7 +18,9 @@ set(CXX_FLAGS
   -Wextra
   -Werror
   -Wconversion
 + -Wno-sign-conversion
   -Wno-unused-parameter
++ -Wno-unused-private-field
   -Wold-style-cast
   -Woverloaded-virtual
-@@ -25,16 +26,15 @@
+  -Wpointer-arith
+@@ -27,16 +29,15 @@ set(CXX_FLAGS
   -march=native
   # -MMD
-  # -std=c++0x
+  -std=c++0x
 - -rdynamic
   )
  if(CMAKE_BUILD_BITS EQUAL 32)