Unbreak MSVC builds after recent Makefile refactoring.

Based on a suggestion by Peter Eisentraut.

src/tools/msvc/pgbison.pl

@@ -42,7 +42,8 @@ open($mf, $makefile);
 local $/ = undef;
 $make = <$mf>;
 close($mf);
-my $headerflag = ($make =~ /\$\(BISON\)\s+-d/ ? '-d' : '');
+my $basetarg = basename($output);
+my $headerflag = ($make =~ /^$basetarg:\s+BISONFLAGS\b.*-d/m ? '-d' : '');
 
 system("bison $headerflag $input -o $output");
 exit $? >> 8;

src/tools/msvc/pgflex.pl

@@ -44,7 +44,8 @@ open($mf, $makefile);
 local $/ = undef;
 $make = <$mf>;
 close($mf);
-my $flexflags = ($make =~ /^\s*FLEXFLAGS\s*=\s*(\S.*)/m ? $1 : '');
+my $basetarg = basename($output);
+my $flexflags = ($make =~ /^$basetarg:\s*FLEXFLAGS\s*=\s*(\S.*)/m ? $1 : '');
 
 system("flex $flexflags -o$output $input");
 if ($? == 0)