pgsql: Enable the use of multiple CPUs/cores when building on MSVC.

Log Message:
-----------
Enable the use of multiple CPUs/cores when building on MSVC. This only
affects the C compiler step - we still only build one target at a
time.

Modified Files:
--------------
pgsql/src/tools/msvc:
Project.pm (r1.19 -> r1.20)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/tools/msvc/Project.pm?r1=1.19&r2=1.20)