mkspecs for msvc should generate pdb files for release builds

A program database (PDB) [1] file holds debugging and project state information that allows incremental linking of a Debug configuration of your program. A PDB file is created when you build with /ZI or /Zi (for C/C++) [2][3][4].

For people familiar with GCC or typical unix compilers, please understand that cl /Zi + link /DEBUG [3] are similar to $(CC) -g, they do not relate to $(CC) -DDEBUG. Because of the way PDB [4] files work, all you have in the final file (.dll/.exe/...) is a reference to a symbol file (.pdb), this is roughly equivalent to .gnu_debuglink [5].

With this change, it's possible to provide pdb files for release versions of Qt. And it would also be possible to maintain and manage a useful symbol server [6].

[1] http://en.wikipedia.org/wiki/Program_database
[2] http://msdn.microsoft.com/en-us/library/958x11bc.aspx
[3] http://msdn.microsoft.com/en-us/library/xe4t6fc1.aspx
[4] http://msdn.microsoft.com/en-us/library/yd4f8bd1.aspx
[5] http://www.totalviewtech.com/support/documentation/tips/gnu_debuglink.html
[6] http://en.wikipedia.org/wiki/Microsoft_Symbol_Server