Work on ANSI C compatibility: modifying the github workflow

The github workflow that attempts to check that OpenSSL ANSI C compatible
defined '_DEFAULT_SOURCE', which effectively turns gcc and clang into a C99
compiler...  perhaps not with regard to pure language features, but it enables
a few too many types and functions that aren't defined in ANSI C library, or
in some cases, in any C language level library.

Instead of '_DEFAULT_SOURCE', this modification defines '_XOPEN_SOURCE=1' and
'_POSIX_SOURCE=200809L', to enable the use of 'timezone', 'ssize_t' and 'strdup()'.

Reviewed-by: Tom Cosgrove <[email protected]>
Reviewed-by: Tomas Mraz <[email protected]>
(Merged from openssl#24173)

.github/workflows/ci.yml

@@ -69,7 +69,7 @@ jobs:
     steps:
     - uses: actions/checkout@v4
     - name: config
-      run: CPPFLAGS=-ansi ./config --banner=Configured no-asm no-makedepend enable-buildtest-c++ enable-fips --strict-warnings -D_DEFAULT_SOURCE && perl configdata.pm --dump
+      run: CPPFLAGS='-ansi -D_XOPEN_SOURCE=1 -D_POSIX_C_SOURCE=200809L' ./config --banner=Configured no-asm no-secure-memory no-makedepend enable-buildtest-c++ enable-fips --strict-warnings && perl configdata.pm --dump
     - name: make
       run: make -s -j4