Correct flag and libs for OpenMP on macOS

Apple's clang on macOS requires the libomp to link. Apple symlinks gcc
to clang, so it can't be detected as clang by executable name

SConstruct

@@ -253,7 +253,15 @@ defaults.noDebugLinkFlags = ''
 defaults.warningFlags = '-Wall'
 defaults.buildPch = False
 env['pch_flags'] = []
-env['openmp_flag'] = '-fopenmp' # used to generate sample build scripts
+env['openmp_flag'] = ['-fopenmp'] # used to generate sample build scripts
+
+env['using_apple_clang'] = False
+# Check if this is actually Apple's clang on macOS
+if env['OS'] == 'Darwin':
+    result = subprocess.check_output([env.subst('$CC'), '--version']).decode('utf-8')
+    if 'clang' in result.lower() and result.startswith('Apple LLVM'):
+        env['using_apple_clang'] = True
+        env['openmp_flag'].insert(0, '-Xpreprocessor')
 
 if 'gcc' in env.subst('$CC') or 'gnu-cc' in env.subst('$CC'):
     defaults.optimizeCcFlags += ' -Wno-inline'
@@ -276,13 +284,13 @@ elif env['CC'] == 'cl': # Visual Studio
     defaults.warningFlags = '/W3'
     defaults.buildPch = True
     env['pch_flags'] = ['/FIpch/system.h']
-    env['openmp_flag'] = '/openmp'
+    env['openmp_flag'] = ['/openmp']
 
 elif 'icc' in env.subst('$CC'):
     defaults.cxxFlags = '-std=c++0x'
     defaults.ccFlags = '-vec-report0 -diag-disable 1478'
     defaults.warningFlags = '-Wcheck'
-    env['openmp_flag'] = '-openmp'
+    env['openmp_flag'] = ['-openmp']
 
 elif 'clang' in env.subst('$CC'):
     defaults.ccFlags = '-fcolor-diagnostics'

samples/cxx/SConscript

@@ -18,6 +18,9 @@ for subdir, name, extensions, openmp in samples:
     localenv = env.Clone()
     if openmp:
         localenv.Append(CXXFLAGS=env['openmp_flag'], LINKFLAGS=env['openmp_flag'])
+        if env['using_apple_clang']:
+            localenv.Append(LIBS=['omp'])
+
         localenv['cmake_extra'] = """
 find_package(OpenMP REQUIRED)
 set(CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS})
@@ -29,6 +32,8 @@ set(CMAKE_EXE_LINKER_FLAGS ${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS})
     if env["OS"] == "Darwin":
         localenv["cmake_extra"] += "find_library(ACCELERATE_FRAMEWORK Accelerate)"
 
+    localenv.Append(LIBS=env['cantera_libs'])
+
     buildSample(localenv.Program, pjoin(subdir, name),
                 mglob(localenv, subdir, *extensions),
                 CPPPATH=['#include', env['boost_inc_dir']])