[configure.ac] spell out RFC 5322 "date" format for POSIX-conforming "date" with no extension.

Following to the advice by Daniel Pouzzner (see the discussion in the issue #7874),
no need to invoke "env" command to set LC_TIME.

* IEEE Std 1003.1-2024 does not request the "-R" option:
	https://pubs.opengroup.org/onlinepubs/9799919799/

* Default "date" in Solaris 11.4 does not support "-R":
	https://docs.oracle.com/cd/E88353_01/html/E37839/date-1.html
* Default "date" in HP-UX 11.22 does not support "-R":
	https://man.freebsd.org/cgi/man.cgi?query=date&apropos=0&sektion=0&manpath=HP-UX+11.22&arch=default&format=html
* Default "date" in AIX 7 does not support "-R":
	https://www.ibm.com/docs/en/aix/7.3?topic=d-date-command

configure.ac

@@ -69,7 +69,7 @@ AS_IF([ test -n "$CFLAG_VISIBILITY" ], [
        AM_CFLAGS="$AM_CFLAGS $CFLAG_VISIBILITY"
        ])
 
-WOLFSSL_BUILD_DATE=$(date -R)
+WOLFSSL_BUILD_DATE=$(LC_TIME=C date +"%a, %d %b %Y %T %z")
 AC_SUBST([WOLFSSL_BUILD_DATE])