LexerParser: Do not override existing _POSIX_C_SOURCE definition

In commit f034b0f663 (CMake compilation: do not use compiler extensions,
2020-03-14, v3.18.0-rc1~494^2) we explicitly defined `_POSIX_C_SOURCE`
to ensure availability of POSIX APIs even when compiler extensions are
not enabled.  Update the code to avoid redefining `_POSIX_C_SOURCE` if
it is already defined.  This occurs when building our release binaries
as configured in `Utilities/Release/linux`, where we define
`_POSIX_C_SOURCE` explicitly on the compiler command line.

Source/cmStandardLexer.h

@@ -7,7 +7,7 @@
 /* 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