support for link-time option in #version

Changes to be committed:
	modified:   src/version.c
	modified:   sys/share/cppregex.cpp
	modified:   sys/share/pmatchregex.c
	modified:   sys/share/posixregex.c
	modified:   util/makedefs.c

Some options in 3.6.0 are determined by what you link with.
The choice of regex support is one.
Let #version show that linked option along with the compile-time options.

sys/share/cppregex.cpp

@@ -10,6 +10,8 @@
 extern "C" {
   #include <hack.h>
 
+  char regex_id[] = "cppregex";
+
   struct nhregex {
     std::unique_ptr<std::regex> re;
     std::unique_ptr<std::regex_error> err;