travis: build with clang-6.0 instead of clang-4.0

Summary:
folly does not compile with clang versions older than 6.0 any more.

This also changes the travis.yml file to set CC and CXX in the environment
so that folly and gmock get built with the intended compiler instead of being
compiled with the default system compiler (gcc 4.8.4 on Ubuntu trusty).  folly
no longer compiles with gcc 4.8 either.

Reviewed By: phoad

Differential Revision: D10435312

fbshipit-source-id: 11eab6d229668fbdad44e3c6cf7bbb82cf175e8a

Author: simpkins

.travis.yml

@@ -37,22 +37,18 @@ addons:
 
 matrix:
   include:
-    # Set COMPILER environment variable instead of CC or CXX because the latter
-    # are overriden by Travis. Setting the compiler in Travis doesn't work
-    # either because it strips version.
-
-    - env: COMPILER=clang-4.0
+    - env: COMPILER_EVAL="CC=clang-6.0 CXX=clang++-6.0"
       addons:
         apt:
           sources:
             - *common_srcs
-            - llvm-toolchain-trusty-4.0
+            - llvm-toolchain-trusty-6.0
           packages:
             - *common_deps
-            - clang-4.0
+            - clang-6.0
             - libstdc++-4.9-dev
 
-    - env: COMPILER=gcc-5
+    - env: COMPILER_EVAL="CC=gcc-5 CXX=g++-5"
       addons:
         apt:
           sources:
@@ -61,7 +57,7 @@ matrix:
             - *common_deps
             - g++-5
 
-    - env: COMPILER=gcc-6
+    - env: COMPILER_EVAL="CC=gcc-6 CXX=g++-6"
       addons:
         apt:
           sources:
@@ -93,11 +89,12 @@ before_script:
   # Install lcov to coveralls conversion + upload tool.
   - gem install coveralls-lcov
   - lcov --version
+  - eval "$COMPILER_EVAL"
 
 script:
   - mkdir build
   - cd build
-  - cmake -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DRSOCKET_CC=$COMPILER
+  - cmake -DCMAKE_BUILD_TYPE=$BUILD_TYPE
           -DRSOCKET_ASAN=$ASAN -DRSOCKET_INSTALL_DEPS=True
           -DRSOCKET_BUILD_WITH_COVERAGE=ON ..
   - make -j4

CMakeLists.txt

@@ -1,19 +1,5 @@
 cmake_minimum_required(VERSION 3.2)
 
-# The RSOCKET_CC CMake variable specifies the C compiler, e.g. gcc-4.9.
-# The C++ compiler name is obtained by replacing "gcc" with "g++" and "clang"
-# with "clang++"". If RSOCKET_CC is not given, the compiler is detected
-# automatically.
-if (RSOCKET_CC)
-  set(ENV{CC} ${RSOCKET_CC})
-  if (${RSOCKET_CC} MATCHES clang)
-    string(REPLACE clang clang++ CXX ${RSOCKET_CC})
-  else ()
-    string(REPLACE gcc g++ CXX ${RSOCKET_CC})
-  endif ()
-  set(ENV{CXX} ${CXX})
-endif ()
-
 project(ReactiveSocket)
 
 # CMake modules.