configure.ac: Drop broken unused check for ptsname_r()

AC_CHECK_FUNCS() defines HAVE_* variables, but AC_CHECK_FUNC()
does not.

Anyway: We do not have any code using HAVE_PTSNAME_R, so
just remove it.

buildlib/config.h.in

@@ -37,9 +37,6 @@
 #undef HAVE_SETRESUID
 #undef HAVE_SETRESGID
 
-/* Check for ptsname_r() */
-#undef HAVE_PTSNAME_R
-
 /* Define to the size of the filesize containing structures */
 #undef _FILE_OFFSET_BITS
 

configure.ac

@@ -188,9 +188,6 @@ AC_SUBST(HAVE_GETRESGID)
 AC_SUBST(HAVE_SETRESUID)
 AC_SUBST(HAVE_SETRESGID)
 
-AC_CHECK_FUNC(ptsname_r)
-AC_SUBST(HAVE_PTSNAME_R)
-
 dnl Check for doxygen
 AC_PATH_PROG(DOXYGEN, doxygen)