curl: allow building without the .pdb package
curl: allow building without the .pdb package

To build the `.pdb` files, we require not only `cv2pdb` but that tool in
turn requires access to a couple of Visual Studio's `.dll` files.

For users who do not have Visual Studio installed, or where `cv2pdb`
_still_ cannot find the `.dll` files in question, let's offer a way out:
by setting the environment variable `WITHOUT_PDBS` to a non-empty
string, they can skip generating the `.pdb` files.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

git-for-windows/MINGW-packages,git-for-windows/MINGW-packages,dscho/MINGW-packages,git-for-windows/MINGW-packages,dscho/MINGW-packages,dscho/MINGW-packages,dscho/MINGW-packages,dscho/MINGW-packages