Geotiff build for windows is broken for shared libraries

[adamstylinski]

There is a simple preprocessor definition that is absent from the makefile.vc file. I spent a painstaking amount of time figuring out what was wrong, as the declspec(dllexport) win32 call does not occur without this. The simplest solution was just to append a /DBUILD_AS_DLL to the CC variable. A comment should at least be given inside the file to remind the user to define this variable if building a shared library. This affects 1.3 but also the 1.4-RC's. I haven't checked SVN.