Adding a fast-math test.

.github/workflows/ubuntu20-fastmath.yml

@@ -0,0 +1,18 @@
+name: Ubuntu 20.04 CI (GCC 9)
+
+on: [push, pull_request]
+
+jobs:
+  ubuntu-build:
+    runs-on: ubuntu-20.04
+    strategy:
+      fail-fast: false
+    steps:
+      - uses: actions/checkout@v3
+      - name: Use cmake
+        run: |
+          mkdir build &&
+          cd build &&
+          cmake -DCMAKE_CXX_FLAGS="-ffast-math" -DFASTFLOAT_TEST=ON ..  &&
+          cmake --build .   &&
+          ctest --output-on-failure

tests/CMakeLists.txt

@@ -9,7 +9,7 @@ option(SYSTEM_DOCTEST "Use system copy of doctest" OFF)
 if (NOT SYSTEM_DOCTEST)
   FetchContent_Declare(doctest
     GIT_REPOSITORY https://github.com/onqtam/doctest.git
-    GIT_TAG 2.4.6)
+    GIT_TAG v2.4.9)
 endif()
 FetchContent_Declare(supplemental_test_files
   GIT_REPOSITORY https://github.com/fastfloat/supplemental_test_files.git