expat: Activate POSIX APIs even without compiler extensions

Compile with the preprocessor definitions necessary for the `arc4random`
family so it's available even when compiler extensions are not enabled.
Similar things are done in cmbzip2, cmcurl, cmlibarchive, cmliblizma and
cmlibuv.

This issue surfaced on a nightly bot after glibc 2.36 added arc4random
functions. cmlibarchive defines the necessary macro, but also relies on
`HAVE_ARC4RANDOM_BUF`.  cmlibarchive's check with the necessary macro
defined was skipped due to cmexpat running the same check before, but
without the macros, and it being cached.

This extends commit c7c3e39e4f (Utilities: Activate POSIX APIs even
without compiler extensions, 2022-06-02, v3.24.0-rc1~34^2) to cover our
build of expat too.

Issue: #20454

Utilities/cmexpat/CMakeLists.txt

@@ -6,6 +6,12 @@ ELSEIF(CMAKE_C_COMPILER_ID STREQUAL "PathScale")
   SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -woffall")
 ENDIF()
 
+# Activate POSIX APIs.
+if(CMAKE_SYSTEM_NAME MATCHES "^(Linux)$")
+  add_definitions(-D_DEFAULT_SOURCE -D_BSD_SOURCE)
+  string(APPEND CMAKE_REQUIRED_DEFINITIONS " -D_DEFAULT_SOURCE -D_BSD_SOURCE")
+endif()
+
 include(ConfigureChecks.cmake)
 configure_file(expat_config.h.cmake expat_config.h @ONLY)