Bring Meson build definition more in line with CMake

FILES.md

@@ -232,7 +232,22 @@ The file can be updated by calling
 make BUILD.bazel
 ```
 
-### `meson.build`
+### `meson.build` and `meson_options.txt`
+
+Meson build definitions suitable for use as a subproject ("wrap" in Meson terminology).
+
+Projects wishing to use the wrap can execute:
+```sh
+meson wrap install nlohmann_json
+```
+
+Which allows Meson to build from source when a system provided dependency isn't available.
+
+To build directly:
+```sh
+meson setup builddir
+ninja -C builddir
+```
 
 ### `Package.swift`
 

meson.build

@@ -2,23 +2,39 @@ project('nlohmann_json',
     'cpp',
     version : '3.12.0',
     license : 'MIT',
+    meson_version : '>= 0.64',
+    default_options: ['cpp_std=c++11'],
 )
 
-nlohmann_json_dep = declare_dependency(
-    include_directories: include_directories('single_include')
-)
+if get_option('MultipleHeaders')
+    incdir = 'include'
+else
+    incdir = 'single_include'
+endif
 
-nlohmann_json_multiple_headers = declare_dependency(
-    include_directories: include_directories('include')
+cpp_args = [
+    '-DJSON_USE_GLOBAL_UDLS=@0@'.format(
+        (not get_option('GlobalUDLs')).to_int()),
+    '-DJSON_USE_IMPLICIT_CONVERSIONS=@0@'.format(
+        (not get_option('ImplicitConversions')).to_int()),
+]
+
+nlohmann_json_dep = declare_dependency(
+    compile_args: cpp_args,
+    include_directories: include_directories(incdir)
 )
+meson.override_dependency('nlohmann_json', nlohmann_json_dep)
 
 if not meson.is_subproject()
-install_headers('single_include/nlohmann/json.hpp', subdir: 'nlohmann')
-install_headers('single_include/nlohmann/json_fwd.hpp', subdir: 'nlohmann')
+    install_subdir(
+        incdir / 'nlohmann',
+        install_dir: get_option('includedir'),
+        install_tag: 'devel',
+    )
 
-pkgc = import('pkgconfig')
-pkgc.generate(name: 'nlohmann_json',
-    version: meson.project_version(),
-    description: 'JSON for Modern C++'
-)
+    pkgc = import('pkgconfig')
+    pkgc.generate(name: 'nlohmann_json',
+        version: meson.project_version(),
+        description: 'JSON for Modern C++'
+    )
 endif

meson_options.txt

@@ -0,0 +1,18 @@
+option(
+    'MultipleHeaders',
+    type: 'boolean',
+    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',
+)
+option(
+    'ImplicitConversions',
+    type: 'boolean',
+    value: true,
+    description: 'Enable implicit conversions',
+)