do not hardcode path to perl

When cross-compiling on other Unix systems like
FreeBSD 10.x, perl is not in /usr/bin. Use env
to find the perl interpreter and use no fixed path
when executing via make.

test/math/Makefile.in

@@ -28,7 +28,7 @@ endif
 EXTRA_CFLAGS    := -fno-builtin
 EXTRA_LDFLAGS   := -lm
 
-PERL := /usr/bin/perl
+PERL := perl
 
 MDEPS := $(wildcard test-*.c)
 $(MDEPS): libm-test.c

test/math/gen-libm-test.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -w
+#!/usr/bin/env perl
 # Copyright (C) 1999 Free Software Foundation, Inc.
 # This file is part of the GNU C Library.
 # Contributed by Andreas Jaeger <aj@suse.de>, 1999.