Building pwlib with bison-1.875-2 fails, because it uses the bison parser with g++.
The following code in yacc.c fails:

/*----------------------------------------------------.
| yyerrlab1 -- error raised explicitly by an action.  |
`----------------------------------------------------*/
yyerrlab1:

  /* Suppress GCC warning that yyerrlab1 is unused when no action
     invokes YYERROR.  */
#if defined (__GNUC_MINOR__) && 2093 <= (__GNUC__ * 1000 + __GNUC_MINOR__)
  __attribute__ ((__unused__))
#endif

]b4_location_if([  yylerrsp = yylsp;
  *++yylerrsp = yyloc;])[
  goto yyerrlab2;


since __attribute__ ((__unused__)) doesn't work in C++. I propose adding
!defined (_CPLUSPLUS_) or something.