build: remove signbit check in configure.ac

We now have a fallback macro in imports.h
This reverts part of 0f3ba405.

Reviewed-by: Matt Turner <mattst88@gmail.com>

configure.ac

@@ -499,13 +499,6 @@ AC_SUBST([DLOPEN_LIBS])
 dnl See if posix_memalign is available
 AC_CHECK_FUNC([posix_memalign], [DEFINES="$DEFINES -DHAVE_POSIX_MEMALIGN"])
 
-dnl signbit() is a macro in glibc's math.h, so AC_CHECK_FUNC fails. To handle
-dnl this, use AC_CHECK_DECLS and fallback to AC_CHECK_FUNC in case it fails.
-AC_CHECK_DECLS([signbit],[],
-               AC_CHECK_FUNC([signbit],[],
-                             AC_MSG_ERROR([could not find signbit()])),
-               [#include <math.h>])
-
 dnl SELinux awareness.
 AC_ARG_ENABLE([selinux],
     [AS_HELP_STRING([--enable-selinux],