Use compiler from DOLFINConfig.cmake file

[Prof Garth Wells]

DOLFIN demos and JIT compilation should use

DOLFIN_CXX_COMPILER

from DOLFINConfig.cmake.

Using the default compiler can fail because of incompatible flags.

[Johan Hake]

Issue #113 was marked as a duplicate of this issue.

[Johan Hake]

The generated CMakeList.txt file used for JIT compilation of dolfin modules has:

if (NOT $ENV{CXX})
  set(CMAKE_CXX_COMPILER ${DOLFIN_CXX_COMPILER})
endif()

so for example all Expressions are compiled using the correct compiler. For JIT compiled UFC modules we have:

if (NOT $ENV{CXX})
  set(CMAKE_CXX_COMPILER ${UFC_CXX_COMPILER})
endif()

so that is not using DOLFIN_CXX_COMPILER. That saidin Python is UFC forms compiled without any bindings to DOLFIN, so I guess that should also be OK, right?

[Prof Garth Wells]

There is something wrong because on Mac I compile DOLFIN using gcc, and then always have to set CXX to gcc, otherwise JIT compilation uses clang, which breaks on the OpenMP flags since clang does not support OpenMP.

I don't have the Mac with me right now to test.

[Johan Hake]

Ok, this should work, as it was the one reason we switched to CMake for JIT.

[Prof Garth Wells]

It seems that

if (NOT $ENV{CXX})
  set(CMAKE_CXX_COMPILER ${UFC_CXX_COMPILER})
endif()

was not working. I have seen that position in the CMake file is important. Anyway, a cleaner has been implemented (CMake devs recommend not setting CMAKE_CXX_COMPILER in CMakeLists.txt files) with DOLFIN_CXX_FLAGS no longer used. This has been made possible by not including CGAL header files via the DOLFIN public interface. Using DOLFIN_CXX_FLAGS is dangerous because they may be compiler-specific.