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

00:09:36

passed cppcheck #13873
Linux MultiThreads

00:34:31

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

00:03:50

passed VS2013-Qt-5.5.1-x64 #13876
VS2013 Qt551

00:15:13

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

00:21:21

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

00:17:56