Don't call clang++ with unsupported -frounding-math.

This avoids a flood of warnings, and CGAL only needs -frounding-math
when compiling with GCC.
6 jobs for clang-no-frounding-math in 34 minutes and 33 seconds
Status Job ID Name Coverage
  Test
passed #13876
Qt551 VS2013
VS2013-Qt-5.5.1-x64

00:15:13

passed #13875
Qt56 VS2015
VS2015-Qt-5.6.0-x32

00:21:21

passed #13874
Qt56 VS2015
VS2015-Qt-5.6.0-x64

00:17:56

passed #13872
MultiThreads Linux
clang-c++11-Qt5

00:09:36

passed #13873
MultiThreads Linux
cppcheck

00:34:31

passed #13871
Linux
gcc-c++11-Qt5

00:03:50