pgsql: Rethink the idea of having plpgsql depend on parser/gram.h.

Log Message:
-----------
Rethink the idea of having plpgsql depend on parser/gram.h.  Aside from the
fact that this is breaking the MSVC build, it's probably not really a good
idea to expand the dependencies of gram.h any further than the core parser;
for instance the value of SCONST might depend on which bison version you'd
built with.  Better to expose an additional call point in parser.c, so
move what I had put into pl_funcs.c into parser.c.  Also PGDLLIMPORT'ify
the reference to standard_conforming_strings, per buildfarm results.

Modified Files:
--------------
    pgsql/src/backend/parser:
        parser.c (r1.76 -> r1.77)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parser.c?r1=1.76&r2=1.77)
    pgsql/src/include/parser:
        parser.h (r1.24 -> r1.25)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/parser.h?r1=1.24&r2=1.25)
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.122 -> r1.123)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.122&r2=1.123)
        pl_funcs.c (r1.77 -> r1.78)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_funcs.c?r1=1.77&r2=1.78)
        plpgsql.h (r1.111 -> r1.112)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h?r1=1.111&r2=1.112)
        scan.l (r1.68 -> r1.69)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/scan.l?r1=1.68&r2=1.69)