Remove -std=c99 from default CI builds

Our CI was using -std=c99 for all builds, which causes Darwin headers
to hide BSD extension symbols. This masked the bswap32/bswap64 macro
collision reported in #419, since real-world users (e.g. Homebrew) build
without -std=c99 and see the conflict.

Remove -std=c99 from the default CFLAGS so the main CI matrix matches
what downstream consumers build with. Add a dedicated strict-C99 matrix
entry (ubuntu/gcc) to preserve language compliance checking.

Also change CMAKE_C_EXTENSIONS from OFF to ON to match default CMake
behavior and what users get when building with cmake.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: oschwald

.github/workflows/test.yml

@@ -12,6 +12,10 @@ jobs:
         os: [ubuntu-latest, macos-latest]
         cc: [gcc, clang]
         posix: ['', -D_POSIX_C_SOURCE=200112L]
+        include:
+          - os: ubuntu-latest
+            cc: gcc
+            posix: '-std=c99'
     name: Autotools build on ${{matrix.os}} using ${{matrix.cc}} ${{matrix.posix}}
     runs-on: ${{ matrix.os }}
     env:
@@ -29,7 +33,7 @@ jobs:
       - run: ./bootstrap
       - run: ./configure
         env:
-          CFLAGS: -std=c99 -Wall -Wextra -Werror -Wno-unused-function -Wno-unused-parameter ${{ matrix.posix }}
+          CFLAGS: -Wall -Wextra -Werror -Wno-unused-function -Wno-unused-parameter ${{ matrix.posix }}
       - run: make
       - run: make check