change the default for gcc 9.x and beyond to not enable OpenMP by def…

…ault even if it is found to be supported

this is so that using CMake by default will compile LAMMPS, since gcc 9.x
expects different sharing semantics for constants than previous versions.

cmake/CMakeLists.txt

@@ -316,7 +316,18 @@ pkg_depends(USER-SCAFACOS MPI)
 
 include(CheckIncludeFileCXX)
 find_package(OpenMP QUIET)
-option(BUILD_OMP "Build with OpenMP support" ${OpenMP_FOUND})
+
+# TODO: this is a temporary workaround until a better solution is found. AK 2019-05-30
+# GNU GCC 9.x uses settings incompatible with our use of 'default(none)' in OpenMP pragmas
+# where we assume older GCC semantics. For the time being, we disable OpenMP by default
+# for GCC 9.x and beyond. People may manually turn it on, but need to run the script
+# src/USER-OMP/hack_openmp_for_pgi_gcc9.sh on all sources to make it compatible with gcc 9.
+if ((${CMAKE_CXX_COMPILER_ID} STREQUAL "GNU") AND (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 9.0.0))
+  option(BUILD_OMP "Build with OpenMP support" OFF)
+else()
+  option(BUILD_OMP "Build with OpenMP support" ${OpenMP_FOUND})
+endif()
+
 if(BUILD_OMP)
   find_package(OpenMP REQUIRED)
   check_include_file_cxx(omp.h HAVE_OMP_H_INCLUDE)