build(omp): added explicit omp prefix override

this is useful when building omp compatible wheels on mac which bundle libomp with them

meson_options.txt

@@ -13,3 +13,4 @@ option('openmp_support', type: 'boolean', value: false, description: 'Enable Ope
 option('use_mimalloc', type: 'boolean', value: true, description: 'Use mimalloc as the memory allocator for GridFire. Generally this is ~10% faster than the system allocator.')
 option('build_benchmarks', type: 'boolean', value: false, description: 'build the benchmark suite')
 option('asan', type: 'boolean', value: false, description: 'Enable AddressSanitizer (ASan) support for detecting memory errors')
+option('libomp_prefix', type: 'string', value: '', description: 'Explicit prefix of an LLVM OpenMP runtime to use instead of auto-detection (macOS wheel builds). Empty = dependency(\'openmp\') auto-detection.')

src/meson.build

@@ -57,7 +57,16 @@ if get_option('plugin_support')
 endif
 
 if get_option('openmp_support')
+    libomp_prefix = get_option('libomp_prefix')
+    if libomp_prefix != ''
+        omp_lib = cpp.find_library('omp', dirs: [libomp_prefix + '/lib'])
+        openmp_dep = declare_dependency(
+            compile_args: ['-Xpreprocessor', '-fopenmp', '-I' + libomp_prefix + '/include'],
+            dependencies: omp_lib,
+        )
+    else
         openmp_dep = dependency('openmp', required: true)
+    endif
     gridfire_build_dependencies += [openmp_dep]
 endif