Add support for C++ in Configure

A note: this will form object file names by changing '.cc' to
'_cc.o'.  This will permit other configuration code to recognise these
object files were built for C++ rather than C.

Reviewed-by: Rich Salz <rsalz@openssl.org>

Configurations/README

diff --git a/Configurations/README b/Configurations/README
index da64e8c..0b82ded 100644
--- a/Configurations/README
+++ b/Configurations/README
@@ -17,12 +17,20 @@
         sys_id          => System identity for systems where that
                            is difficult to determine automatically.
 
-        cc              => The compiler command, usually one of "cc",
+        cc              => The C compiler command, usually one of "cc",
                            "gcc" or "clang".  This command is normally
                            also used to link object files and
                            libraries into the final program.
+        cxx             => The C++ compiler command, usually one of
+                           "c++", "g++" or "clang++".  This command is
+                           also used when linking a program where at
+                           least one of the object file is made from
+                           C++ source.
         cflags          => Flags that are used at all times when
-                           compiling.
+                           compiling C object files.
+        cxxflags        => Flags that are used at all times when
+                           compiling C++ object files.  If unset, it
+                           gets the same value as cflags.
         defines         => As an alternative, macro definitions may be
                            present here instead of in `cflags'.  If
                            given here, they MUST be as an array of the