lib/Makefile.m32: allow customizing dll suffixes

- New `CURL_DLL_SUFFIX` envvar will add a suffix to the generated
  libcurl dll name. Useful to add `-x64` to 64-bit builds so that
  it can live in the same directory as the 32-bit one. By default
  this is empty.

- New `CURL_DLL_A_SUFFIX` envvar to customize the suffix of the
  generated import library (implib) for libcurl .dll. It defaults
  to `dll`, and it's useful to modify that to `.dll` to have the
  standard naming scheme for mingw-built .dlls, i.e. `libcurl.dll.a`.

Closes https://github.com/curl/curl/pull/1942