Merge topic 'lexer-posix-source' into release-3.19

a8b41e2c68 LexerParser: Do not override existing _POSIX_C_SOURCE definition
82cfeeb484 Merge branch 'backport-3.18-ci-wix-download' into release-3.18

Acked-by: Kitware Robot <[email protected]>
Merge-request: !5776

Source/cmStandardLexer.h

@@ -6,7 +6,7 @@
 /* Needed for glibc < 2.12 */
 #  define _XOPEN_SOURCE 600
 #endif
-#if !defined(_WIN32) && !defined(__sun)
+#if !defined(_POSIX_C_SOURCE) && !defined(_WIN32) && !defined(__sun)
 /* POSIX APIs are needed */
 #  define _POSIX_C_SOURCE 200809L
 #endif