Implement support for digit separators in preprocessor expressions

Integers with digit separators are already recognized as literal
tokens. This commit adds proper interpretation of them as integers for
use in expressions e.g. with #if

Author: jefftrull

include/boost/wave/cpplexer/re2clex/cpp.re

@@ -468,7 +468,7 @@ pp_number:
 
     if (s->detect_pp_numbers) {
     /*!re2c
-        "."? Digit (Digit | NonDigit | ExponentStart | ".")*
+        "."? Digit (("'"? (Digit | NonDigit | ExponentStart)) | ".")*
             { BOOST_WAVE_RET(T_PP_NUMBER); }
 
         // because we reached this point, then reset the cursor,