[SCons] Disable OMP on XCode Clang

According to several sources online, XCode does not support OMP. This
disables checking for OMP support when Apple's clang (XCode) is being
used.

In addition, the library associated with GCC is called gomp. This fixes
the CI to run the samples correctly.

.github/workflows/main.yml

@@ -36,7 +36,7 @@ jobs:
     - name: Install Apt dependencies
       run: |
         sudo apt update
-        sudo apt install libboost-dev gfortran libopenmpi-dev
+        sudo apt install libboost-dev gfortran libopenmpi-dev libomp-dev
     - name: Upgrade pip
       run: python3 -m pip install -U pip setuptools wheel
     - name: Install Python dependencies

SConstruct

@@ -1164,7 +1164,11 @@ else:
     env['cxx_stdlib'] = []
 
 env['HAS_CLANG'] = conf.CheckDeclaration('__clang__', '', 'C++')
-env['HAS_OPENMP'] = conf.CheckLibWithHeader("omp", "omp.h", language="C++")
+if not env["using_apple_clang"]:
+    env['HAS_OPENMP'] = conf.CheckLibWithHeader("gomp", "omp.h", language="C++")
+else:
+    env["HAS_OPENMP"] = False
+    logger.info("Not checking for OpenMP support due to using XCode compiler.")
 
 boost_version_source = get_expression_value(['<boost/version.hpp>'], 'BOOST_LIB_VERSION')
 retcode, boost_lib_version = conf.TryRun(boost_version_source, '.cpp')