Document the Meson options and check them in CI

Describe the options of meson.build in FILES.md and in the Meson section
of the package manager documentation, which so far said that a Meson
installation always uses the default configuration.

The ci_meson_install job now also installs with non-default options and
checks that the headers of the multi-header version are installed, that
the definitions reach the pkg-config file and the CMake target, and that
find_package still works.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/ubuntu.yml

@@ -49,6 +49,20 @@ jobs:
         run: |
           cmake -S tests/cmake_import/project -B build-import -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install
           cmake --build build-import
+      - name: Install with Meson and non-default options
+        run: |
+          meson setup build-meson-options --prefix=${{ github.workspace }}/install-options -DMultipleHeaders=true -DDiagnostics=true -DGlobalUDLs=false
+          meson install -C build-meson-options
+      - name: Check that the options reach the installed files
+        run: |
+          test -d install-options/include/nlohmann/detail
+          cflags=$(PKG_CONFIG_PATH=${{ github.workspace }}/install-options/share/pkgconfig pkg-config --cflags nlohmann_json)
+          echo "$cflags"
+          echo "$cflags" | grep -q -- '-DJSON_DIAGNOSTICS=1'
+          echo "$cflags" | grep -q -- '-DJSON_USE_GLOBAL_UDLS=0'
+          grep -q 'JSON_USE_GLOBAL_UDLS=0;JSON_DIAGNOSTICS=1' install-options/share/cmake/nlohmann_json/nlohmann_jsonTargets.cmake
+          cmake -S tests/cmake_import/project -B build-import-options -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install-options
+          cmake --build build-import-options
 
   ci_infer:
     runs-on: ubuntu-latest