Remove -std=c99 from default CI builds, add macOS 26

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 and add it as a separate matrix
entry to preserve language compliance checking. Also add macos-26 to the
OS matrix since the bswap collision appears to be specific to macOS 26
(Tahoe), which is not covered by macos-latest (currently macOS 15).

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

Author: oschwald

.github/workflows/test.yml

@@ -9,9 +9,9 @@ jobs:
   test-autoconf:
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-latest, macos-latest, macos-26]
         cc: [gcc, clang]
-        posix: ['', -D_POSIX_C_SOURCE=200112L]
+        posix: ['', -D_POSIX_C_SOURCE=200112L, -std=c99]
     name: Autotools build on ${{matrix.os}} using ${{matrix.cc}} ${{matrix.posix}}
     runs-on: ${{ matrix.os }}
     env:
@@ -23,13 +23,13 @@ jobs:
           submodules: true
           persist-credentials: false
       - run: sudo apt install libipc-run3-perl
-        if: ${{ matrix.os == 'ubuntu-latest' }}
+        if: ${{ startsWith(matrix.os, 'ubuntu') }}
       - run: brew install autoconf automake libtool
-        if: ${{ matrix.os == 'macos-latest' }}
+        if: ${{ startsWith(matrix.os, 'macos') }}
       - 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