Only provide execvpe(3) when needed

Check for the existence of execvpe(3) and only provide the FreeBSD
compat version if required.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Brooks Davis <brooks.davis@sri.com>
Closes #15609

lib/libspl/include/os/freebsd/sys/param.h

@@ -57,6 +57,8 @@
 extern size_t spl_pagesize(void);
 #define	PAGESIZE	(spl_pagesize())
 
+#ifndef HAVE_EXECVPE
 extern int execvpe(const char *name, char * const argv[], char * const envp[]);
+#endif
 
 #endif