Test case:

[1] touch hotspot/make/closed/linux/makefiles/mapfile-ext
[2] Try to rebuild hotspot

=> libjvm.so is NOT updated

The fix is here for linux. Fixes for other platforms are similar.

=========================
diff -r cca27510c1d5 make/linux/makefiles/vm.make
--- a/make/linux/makefiles/vm.make Tue Aug 18 11:27:23 2015 -0700
+++ b/make/linux/makefiles/vm.make Mon Aug 31 10:14:54 2015 -0700
@@ -259,12 +259,18 @@
  cat $(VM_DEF_EXT) >> $@
 endif
 
+MAPFILE_EXT_SRC = $(HS_ALT_MAKE)/linux/makefiles/mapfile-ext
+
+ifneq ("$(wildcard $(MAPFILE_EXT_SRC))","")
+mapfile_ext: $(MAPFILE_EXT_SRC)
+ rm -f $@
+ $(CP) $(MAPFILE_EXT_SRC) $@
+ touch $@
+else
 mapfile_ext:
  rm -f $@
  touch $@
- if [ -f $(HS_ALT_MAKE)/linux/makefiles/mapfile-ext ]; then \
- cat $(HS_ALT_MAKE)/linux/makefiles/mapfile-ext > $@; \
- fi
+endif
 
 ifeq ($(JVM_VARIANT_ZEROSHARK), true)
   STATIC_CXX = false
===============================