Allow overridding OpenMP flags to empty (trilinos/Trilinos#1400)

I don't have automated tests for this.  Adding automated tests would require
actually running with a compiler with OpenMPI support.  That would make these
tests not very portable.  Therefore, I manually tested this.  But the code is
very simple so it would be hard to break unless you were very sloppy.

Build/Test Cases Summary
Enabled Packages:
Enabled all Packages
0) MPI_DEBUG => passed: passed=265,notpassed=0 (0.80 min)
1) SERIAL_RELEASE => passed: passed=265,notpassed=0 (0.59 min)
2) MPI_DEBUG_CMAKE-3.6.2 => passed: passed=286,notpassed=0 (0.65 min)
3) SERIAL_RELEASE_CMAKE-3.6.2 => passed: passed=286,notpassed=0 (0.50 min)
Other local commits for this build/test group: 24d3975

tribits/core/package_arch/TribitsGlobalMacros.cmake

@@ -1951,10 +1951,10 @@ MACRO(TRIBITS_SETUP_ENV)
   IF(${PROJECT_NAME}_ENABLE_OpenMP)
     FIND_PACKAGE(OpenMP)
     IF(OPENMP_FOUND)
-      SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
-      SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
+      TRIBITS_SET_OPENMP_FLAGS(CXX)
+      TRIBITS_SET_OPENMP_FLAGS(C)
       IF(OpenMP_Fortran_FLAGS)
-        SET(CMAKE_Fortran_FLAGS "${CMAKE_Fortran_FLAGS} ${OpenMP_Fortran_FLAGS}")
+        TRIBITS_SET_OPENMP_FLAGS(Fortran)
       ELSE()
       # Older versions of FindOpenMP.cmake don't find Fortran flags.  Mike H said this is safe.
         SET(CMAKE_Fortran_FLAGS "${CMAKE_Fortran_FLAGS} ${OpenMP_C_FLAGS}")
@@ -2012,6 +2012,16 @@ MACRO(TRIBITS_SETUP_ENV)
 
 ENDMACRO()
 
+
+MACRO(TRIBITS_SET_OPENMP_FLAGS  LANG)
+  IF (NOT "${OpenMP_${LANG}_FLAGS_OVERRIDE}" STREQUAL "")
+    SET(CMAKE_${LANG}_FLAGS "${CMAKE_${LANG}_FLAGS} ${OpenMP_${LANG}_FLAGS_OVERRIDE}")
+  ELSE()
+    SET(CMAKE_${LANG}_FLAGS "${CMAKE_${LANG}_FLAGS} ${OpenMP_${LANG}_FLAGS}")
+  ENDIF()
+ENDMACRO()
+
+
 #
 # Set mapping of labels to subprojects (i.e. TriBITS packages) for local CTest
 # only.

tribits/doc/build_ref/TribitsBuildReferenceBody.rst

@@ -1105,6 +1105,17 @@ To enable OpenMP support, one must set::
 Note that if you enable OpenMP directly through a compiler option (e.g.,
 ``-fopenmp``), you will NOT enable OpenMP inside <Project> source code.
 
+To skip adding flags for OpenMP for ``<LANG>`` = ``C``, ``CXX``, or
+``Fortran``, use::
+
+  -D OpenMP_<LANG>_FLAGS_OVERRIDE=" "
+
+The single space " " will result in no flags getting added.  This is needed
+since one can't set the flags ``OpenMP_<LANG>_FLAGS`` to an empty string or
+the ``FIND_PACKAGE(OpenMP)`` command will fail.  Setting the variable
+``-DOpenMP_<LANG>_FLAGS_OVERRIDE= " "`` is the only way to enable OpenMP but
+skip adding the OpenMP flags provided by ``FIND_PACKAGE(OpenMP)``.
+
 .. _BUILD_SHARED_LIBS:
 
 Building shared libraries