meson: add support for the GlobalUDLs option

Signed-off-by: Dylan Baker <dylan@pnwbakers.com>

meson.build

@@ -12,7 +12,13 @@ else
     incdir = 'single_include'
 endif
 
+cpp_args = [
+    '-DJSON_USE_GLOBAL_UDLS=@0@'.format(
+        (not get_option('GlobalUDLs')).to_int()),
+]
+
 nlohmann_json_dep = declare_dependency(
+    compile_args: cpp_args,
     include_directories: include_directories(incdir)
 )
 meson.override_dependency('nlohmann_json', nlohmann_json_dep)

meson_options.txt

@@ -4,3 +4,9 @@ option(
     value: true,
     description: 'Use non-amalgamated version of the library',
 )
+option(
+    'GlobalUDLs',
+    type: 'boolean',
+    value: true,
+    description: 'Place user-defined string literals in the global namespace',
+)