test for amalgamation

.github/workflows/amalgamate-ubuntu20.yml

@@ -0,0 +1,25 @@
+name: Amalgamate Ubuntu 20.04 CI (GCC 9, 8)
+
+on: [push, pull_request]
+
+jobs:
+  ubuntu-build:
+    runs-on: ubuntu-20.04
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          # Legacy/x86 compilers cause CI failures.
+          #- {cxx: -DCMAKE_CXX_COMPILER=g++-8, arch:                         }
+          - {cxx:                           , arch:                         } # default=gcc9
+          #- {cxx:                           , arch: -DCMAKE_CXX_FLAGS="-m32"} # default=gcc9
+    steps:
+      - uses: actions/checkout@v2
+      - name: Compile with amalgamation
+        run: |
+          mkdir build &&
+          mkdir build/fast_float &&
+          python3 ./scripts/amalgamate.py > build/fast_float/fast_float.h &&
+          cp tests/string_test.cpp build/ &&
+          cd build &&
+          g++ string_test.cpp