restore build on ancient Mac OS X (re: 22e044c3)

For some reason that I can't be bothered to investigate, gcc 3.3 on
my museum-grade PowerMac G5 running Mac OS X 10.3 doesn't like -lm
when compiling the output{...}end blocks in features/pty and
features/dll. The compilation fails silently. But some other
systems require the -lm for it to work.

Thankfully iffe has a syntax for trying a test repeatedly with
different flags (a.k.a. prerequisites). From 'iffe --man':

    -     Prereq grouping mark; prereqs before the first - are
          passed to all feature tests. Subsequent groups are
          attempted in left-to-right order until the first
          successful group is found.

src/cmd/builtin/features/pty,
src/lib/libdll/features/dll:
- Try compiling the output{...}end block once with -lm and, if that
  fails, try once without -lm before erroring out.

src/cmd/builtin/features/pty

@@ -11,7 +11,8 @@ lib	openpty,_getpty,ptsname -lutil
 lib	grantpt,unlockpt,posix_openpt stdlib.h
 lib	cfmakeraw termios.h
 
-tst - -lm output{
+# try once with -lm, once without
+tst - -lm - - output{
 	#include	<fcntl.h>
 	#if _lib_ptsname
 	#include	<stdlib.h>
@@ -50,8 +51,11 @@ tst - -lm output{
 		return 0;
 	}
 }end fail{
-	echo "$0: Output block failed to compile. Export IFFEFLAGS=-d1 to debug." >&2
-	exit 1
+	case ${_features_pty_TRIEDONCE-no} in
+	no)	_features_pty_TRIEDONCE=yes ;;
+	*)	echo "$0: Output block failed to compile. Export IFFEFLAGS=-d1 to debug." >&2
+		exit 1 ;;
+	esac
 }end
 
 extern	_getpty		char*		(int*, int, mode_t, int)

src/lib/libdll/features/dll

@@ -73,7 +73,9 @@ tst	run{
 	esac
 	echo "#define _DLL_NEXT_PATH	\"$lib\""
 }end
-tst	- -lm output{
+
+# try once with -lm, once without
+tst	- -lm - - output{
 	#include <math.h>
 	#if defined(__MVS__) && !defined(__SUSV3)
 	#define __SUSV3		1
@@ -261,6 +263,9 @@ tst	- -lm output{
 		return 0;
 	}
 }end fail{
-	echo "$0: Output block failed to compile. Export IFFEFLAGS=-d1 to debug." >&2
-	exit 1
+	case ${_features_dll_TRIEDONCE-no} in
+	no)	_features_dll_TRIEDONCE=yes ;;
+	*)	echo "$0: Output block failed to compile. Export IFFEFLAGS=-d1 to debug." >&2
+		exit 1 ;;
+	esac
 }end