libquadmath: Restore linking against -lm on most targets [PR112963]

The r14-4825 change added AC_CHECK_LIBM to libquadmath configure.ac and
replaced unconditional linking with -lm with linking with $(LIBM)
determined by that.
Unfortunately that broke bare metal targets because AC_CHECK_LIBM attempts
to link against -lm and this was after (unconditional) GCC_NO_EXECUTABLES.
Then r14-4863 partially reverted that change (no longer AC_CHECK_LIBM),
but didn't revert the Makefile.am change of -lm to $(LIBM), which had
the effect that libquadmath is not linked against -lm on any arch.
That is a serious problem though e.g. on Linux, because libquadmath calls
a few libm entrypoints and e.g. on powerpc64le the underlinking can cause
crashes in IFUNC resolvers of libm.
Instead of adding further reversion of the r14-4825 commit and use -lm
unconditionally again, this patch adds an AC_CHECK_LIBM like substitutions
with the *-ncr-sysv4.3* target handling removed (I think we don't support
such targets, especially not in libquadmath) and with the default case
replaced by simple using -lm.  That is something in between using -lm
unconditionally and what AC_CHECK_LIBM does if it would work on bare metal
- we know from GCC 13 and earlier that we can link -lm on all targets
libquadmath is built for, and just white list a couple of targets which
we know don't have separate -lm and don't want to link against that
(like Darwin, Cygwin, ...).

2023-12-12  Jakub Jelinek  <jakub@redhat.com>

	PR libquadmath/112963
	* configure.ac (LIBM): Readd AC_CHECK_LIBM-like check without doing
	AC_CHECK_LIB in it.
	* configure: Regenerated.
	* Makefile.in: Regenerated.

libquadmath/Makefile.in

@@ -355,6 +355,7 @@ INSTALL_SCRIPT = @INSTALL_SCRIPT@
 INSTALL_STRIP_PROGRAM = @INSTALL_STRIP_PROGRAM@
 LD = @LD@
 LDFLAGS = @LDFLAGS@
+LIBM = @LIBM@
 LIBOBJS = @LIBOBJS@
 LIBS = @LIBS@
 LIBTOOL = @LIBTOOL@

libquadmath/configure

@@ -644,6 +644,7 @@ LIBQUAD_USE_SYMVER_GNU_FALSE
 LIBQUAD_USE_SYMVER_GNU_TRUE
 LIBQUAD_USE_SYMVER_FALSE
 LIBQUAD_USE_SYMVER_TRUE
+LIBM
 toolexeclibdir
 toolexecdir
 MAINT
@@ -10921,7 +10922,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 10924 "configure"
+#line 10925 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -11027,7 +11028,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 11030 "configure"
+#line 11031 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -12260,6 +12261,20 @@ esac
 
 
 
+# AC_CHECK_LIBM variant which avoids AC_CHECK_LIB (that doesn't work
+# on bare metal).  In the past we've used -lm in Makefile.am unconditionally,
+# let's use it there unless target knows it doesn't need that.
+LIBM=
+case $host in
+*-*-beos* | *-*-cegcc* | *-*-cygwin* | *-*-haiku* | *-*-pw32* | *-*-darwin*)
+  # These system don't have libm, or don't need it
+  ;;
+*)
+  LIBM=-lm
+  ;;
+esac
+
+
 for ac_header in fenv.h langinfo.h locale.h wchar.h wctype.h limits.h ctype.h printf.h errno.h
 do :
   as_ac_Header=`$as_echo "ac_cv_header_$ac_header" | $as_tr_sh`

libquadmath/configure.ac

@@ -122,6 +122,20 @@ esac
 AC_SUBST(toolexecdir)
 AC_SUBST(toolexeclibdir)
 
+# AC_CHECK_LIBM variant which avoids AC_CHECK_LIB (that doesn't work
+# on bare metal).  In the past we've used -lm in Makefile.am unconditionally,
+# let's use it there unless target knows it doesn't need that.
+LIBM=
+case $host in
+*-*-beos* | *-*-cegcc* | *-*-cygwin* | *-*-haiku* | *-*-pw32* | *-*-darwin*)
+  # These system don't have libm, or don't need it
+  ;;
+*)
+  LIBM=-lm
+  ;;
+esac
+AC_SUBST([LIBM])
+
 AC_CHECK_HEADERS(fenv.h langinfo.h locale.h wchar.h wctype.h limits.h ctype.h printf.h errno.h)
 LIBQUAD_CHECK_MATH_H_SIGNGAM