Handle spaces for Python install location in MSVC scripts

Attempting to use an installation path of Python that includes spaces
caused the MSVC builds to fail.  This fixes the issue by using the same
quoting method as ad7595b for OpenSSL.

Author: Victor Wagner
Discussion: https://postgr.es/m/20200430150608.6dc6b8c4@antares.wagner.home
Backpatch-through: 9.5

src/tools/msvc/Mkvcbuild.pm

@@ -487,7 +487,7 @@ sub mkvcbuild
 		my $pythonprog = "import sys;print(sys.prefix);"
 		  . "print(str(sys.version_info[0])+str(sys.version_info[1]))";
 		my $prefixcmd =
-		  $solution->{options}->{python} . "\\python -c \"$pythonprog\"";
+		  qq("$solution->{options}->{python}\\python" -c "$pythonprog");
 		my $pyout = `$prefixcmd`;
 		die "Could not query for python version!\n" if $?;
 		my ($pyprefix, $pyver) = split(/\r?\n/, $pyout);