meson: handle single header and multiheader the same way CMake does

This uses a meson option (set in `meson_options.txt`) to control whether
multi-header or single-header setup is wanted.

meson.build

@@ -6,19 +6,20 @@ project('nlohmann_json',
     default_options: ['cpp_std=c++11'],
 )
 
+if get_option('MultipleHeaders')
+    incdir = 'include'
+else
+    incdir = 'single_include'
+endif
+
 nlohmann_json_dep = declare_dependency(
-    include_directories: include_directories('single_include')
+    include_directories: include_directories(incdir)
 )
 meson.override_dependency('nlohmann_json', nlohmann_json_dep)
 
-nlohmann_json_multiple_headers = declare_dependency(
-    include_directories: include_directories('include')
-)
-meson.override_dependency('nlohmann_json_multiple_headers', nlohmann_json_multiple_headers)
-
 if not meson.is_subproject()
     install_subdir(
-        'single_include/nlohmann',
+        incdir / 'nlohmann',
         install_dir: get_option('includedir'),
         install_tag: 'devel',
     )

meson_options.txt

@@ -0,0 +1,6 @@
+option(
+    'MultipleHeaders',
+    type: 'boolean',
+    value: true,
+    description: 'Use non-amalgomated version of the library',
+)