release: always link librt for glibc back-compat builds

glibc absorbed clock_gettime in 2.17. librt (its previous location) is safe to
link in anyway for back-compat.

Fixes #7420

configure.ac

@@ -393,6 +393,10 @@ AX_GCC_FUNC_ATTRIBUTE([dllimport])
 
 if test x$use_glibc_compat != xno; then
 
+  #glibc absorbed clock_gettime in 2.17. librt (its previous location) is safe to link
+  #in anyway for back-compat.
+  AC_CHECK_LIB([rt],[clock_gettime],, AC_MSG_ERROR(lib missing))
+
   #__fdelt_chk's params and return type have changed from long unsigned int to long int.
   # See which one is present here.
   AC_MSG_CHECKING(__fdelt_chk type)
@@ -406,7 +410,8 @@ if test x$use_glibc_compat != xno; then
     [ fdelt_type="long int"])
   AC_MSG_RESULT($fdelt_type)
   AC_DEFINE_UNQUOTED(FDELT_TYPE, $fdelt_type,[parameter and return value type for __fdelt_chk])
-
+else
+  AC_SEARCH_LIBS([clock_gettime],[rt])
 fi
 
 if test x$use_hardening != xno; then
@@ -478,8 +483,6 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/socket.h>]],
  [ AC_MSG_RESULT(no)]
 )
 
-AC_SEARCH_LIBS([clock_gettime],[rt])
-
 AC_MSG_CHECKING([for visibility attribute])
 AC_LINK_IFELSE([AC_LANG_SOURCE([
   int foo_def( void ) __attribute__((visibility("default")));