patch 9.0.0108: configure check for timer_create may give wrong error

Problem:    Configure check for timer_create may give wrong error.
Solution:   Give a warning instead of an error.
diff --git a/src/configure.ac b/src/configure.ac
index d777b05..76eb563 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -3828,7 +3828,7 @@
   ])],
   vim_cv_timer_create=yes,
   vim_cv_timer_create=no,
-  AC_MSG_ERROR(cross-compiling: please set 'vim_cv_timer_create')
+  AC_MSG_WARN([failed to build test program; if cross-compiling please set 'vim_cv_timer_create'])
   )])
 
 dnl If the previous failed, check for timer_create() and linking with -lrt.
@@ -3856,7 +3856,7 @@
       ])],
       vim_cv_timer_create_with_lrt=yes,
       vim_cv_timer_create_with_lrt=no,
-      AC_MSG_ERROR(cross-compiling: please set 'vim_cv_timer_create_with_lrt')
+      AC_MSG_WARN([failed to build test program; if cross-compiling please set 'vim_cv_timer_create_with_lrt'])
     )])
   LIBS="$save_LIBS"
 else