Allow --with-extra-cxxflags to work with HotSpot.

[Ian Rogers]

The following (modest) patch against jdk9dev allows the top-level configure
with --with-extra-cxxflags to pass options to the HotSpot build. An example
would be:

configure --fastdebug --with-extra-cxxflags=-std=c++11

Unless --with-extra-cxxflags is specified the option has no effect. I'd
like to work to get this merged into jdk9dev and would appreciate any help.

Thanks,
Ian Rogers, Google.

---
Allow --with-extra-cxxflags to work with HotSpot.

Pass down EXTRA_CXXFLAGS from gnumake's invocation or environment.

diff -r fff6b54e9770 make/aix/makefiles/vm.make
--- a/make/aix/makefiles/vm.make        Thu Jul 16 19:28:37 2015 -0700
+++ b/make/aix/makefiles/vm.make        Tue Jul 21 08:55:08 2015 -0700
@@ -112,6 +112,7 @@
 # Extra flags from gnumake's invocation or environment
 CFLAGS += $(EXTRA_CFLAGS)
 LFLAGS += $(EXTRA_CFLAGS)
+CXXFLAGS += $(EXTRA_CXXFLAGS)

 # Don't set excutable bit on stack segment
 # the same could be done by separate execstack command
diff -r fff6b54e9770 make/bsd/makefiles/vm.make
--- a/make/bsd/makefiles/vm.make        Thu Jul 16 19:28:37 2015 -0700
+++ b/make/bsd/makefiles/vm.make        Tue Jul 21 08:55:08 2015 -0700
@@ -114,6 +114,7 @@
 # Extra flags from gnumake's invocation or environment
 CFLAGS += $(EXTRA_CFLAGS)
 LFLAGS += $(EXTRA_CFLAGS)
+CXXFLAGS += $(EXTRA_CXXFLAGS)

 # Don't set excutable bit on stack segment
 # the same could be done by separate execstack command
diff -r fff6b54e9770 make/linux/makefiles/vm.make
--- a/make/linux/makefiles/vm.make      Thu Jul 16 19:28:37 2015 -0700
+++ b/make/linux/makefiles/vm.make      Tue Jul 21 08:55:08 2015 -0700
@@ -117,6 +117,7 @@
 # Extra flags from gnumake's invocation or environment
 CFLAGS += $(EXTRA_CFLAGS)
 LFLAGS += $(EXTRA_CFLAGS)
+CXXFLAGS += $(EXTRA_CXXFLAGS)

 # Don't set excutable bit on stack segment
 # the same could be done by separate execstack command
diff -r fff6b54e9770 make/solaris/makefiles/vm.make
--- a/make/solaris/makefiles/vm.make    Thu Jul 16 19:28:37 2015 -0700
+++ b/make/solaris/makefiles/vm.make    Tue Jul 21 08:55:08 2015 -0700
@@ -110,6 +110,7 @@

 # Extra flags from gnumake's invocation or environment
 CFLAGS += $(EXTRA_CFLAGS)
+CXXFLAGS += $(EXTRA_CXXFLAGS)

 # Math Library (libm.so), do not use -lm.
 #    There might be two versions of libm.so on the build system: