Specify nlohmann-json version >= 3.9.0 (fixes #113)

* ordered_json was not available in earlier versions
* Add workflow to test fallback for ubuntu 20.04 which has 3.7.3

.github/workflows/build.yml

@@ -8,8 +8,22 @@ on:
     branches: [ master ]
 
 jobs:
-  build-ubuntu:
-    runs-on: ubuntu-latest
+  build-ubuntu-22-04:
+    runs-on: ubuntu-22.04
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Install dependencies (apt)
+      run: sudo apt install python3-pip ninja-build libsndfile1-dev libliquid-dev nlohmann-json3-dev
+    - name: Install meson (pip3)
+      run: pip3 install --user meson
+    - name: meson setup
+      run: meson setup -Dwerror=true build
+    - name: compile
+      run: cd build && meson compile
+
+  build-ubuntu-20-04:
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v4

meson.build

@@ -39,7 +39,7 @@ endif
 sndfile = dependency('sndfile')
 
 # Find nlohmann's json
-json = dependency('nlohmann_json')
+json = dependency('nlohmann_json', version : '>=3.9.0')
 
 # Find iconv; may require -liconv
 foreach linker_args : [ ['-liconv'], [] ]