Can be removed in FreeBSD 15

--- configure.ac.orig	2025-06-24 10:24:24 UTC
+++ configure.ac
@@ -118,6 +118,8 @@ AC_CHECK_FUNC([timegm], [], [AC_MSG_ERROR([timegm() fu
 # Check for the timegm() function (not part of POSIX / Open Group specs)
 AC_CHECK_FUNC([timegm], [], [AC_MSG_ERROR([timegm() function not found])])
 
+AC_CHECK_FUNCS([clearenv])
+
 # Check for endian headers
 AC_CHECK_HEADERS([endian.h sys/endian.h byteswap.h])