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

lib/libzfs/os/freebsd/libzfs_compat.c

@@ -38,7 +38,8 @@
 #define	ZFS_KMOD	"openzfs"
 #endif
 
-
+#ifndef HAVE_EXECVPE
+/* FreeBSD prior to 15 lacks execvpe */
 static int
 execvPe(const char *name, const char *path, char * const *argv,
     char * const *envp)
@@ -192,6 +193,7 @@ execvpe(const char *name, char * const argv[], char * const envp[])
 
 	return (execvPe(name, path, argv, envp));
 }
+#endif /* !HAVE_EXECVPE */
 
 static __thread char errbuf[ERRBUFLEN];