Add testing of standalone mode.

Allow tests to be built in standalone mode.
Update CI.
Update docs.

test/regress/test_icu.cpp

@@ -20,10 +20,25 @@
 // We can only build this if we have ICU support:
 //
 #include <boost/regex/config.hpp>
+
+#ifdef __has_include
+#if __has_include(<unicode/utypes.h>)
+#define BOOST_HAS_ICU
+#endif
+#endif
+
 #if defined(BOOST_HAS_ICU) && !defined(BOOST_NO_STD_WSTRING)
 
 #include <boost/regex/icu.hpp>
+#ifndef BOOST_REGEX_STANDALONE
 #include <boost/mpl/int.hpp>
+#else
+#include <type_traits>
+namespace boost { namespace mpl {
+   template <int I>
+   using int_ = ::std::integral_constant<int, I>; 
+} }
+#endif
 #include "test.hpp"
 
 namespace unnecessary_fix{