Add basic ingredients for CONDA/cl.exe build for Windows.

[willend]

This PR includes

• Minimal edits to src/mcpl/mcpl.c for missing windows unistd/getopt
• A public domain (?) implementation of unistd.h in src/mcpl (was taken from https://stackoverflow.com/a/826027/1202830 - author gives no explicit license there)
• CMAKE infrastructure to ensure the CONDA include path is explicitly set for cl.exe to pick up
• In util/ I have put a .bat snippet to enable python from the surrounding conda env being run on the bin/mcpl-config python script
• zlib is successfully picked up from CONDA via the above include-workaround

Once this is in place, we should be ready to build a Windows package with enough features for a functional McStas release on Windows / conda via a build.bat with something like

cmake -DCMAKE_INSTALL_PREFIX=%PREFIX%  ^
-S %SRCDIR% -G "NMake Makefiles"  ^
-DBUILD_SHARED_LIBS=ON -DMCPL_DISABLE_CXX=ON ^
-DMCPL_NOTOUCH_CMAKE_BUILD_TYPE=ON  ^
-DMCPL_ENABLE_RPATHMOD=OFF  ^
-DCMAKE_INSTALL_LIBDIR=lib - ^
DCMAKE_BUILD_TYPE=Release ^
-DMCPL_ENABLE_EXAMPLES=OFF  ^
-DMCPL_ENABLE_PYTHON=OFF  ^
-DMCPL_ENABLE_ZLIB=USEPREINSTALLED  ^
-DMCPL_CONDA_WINDOWS=ON  ^
-DCMAKE_WINDOWS_EXPORT_ALL_SYMBOLS=ON  ^
%CMAKE_ARGS%

cmake --build . --target install --config Release

(- Hinges on conda dependencies getopt-win32, zlib etc.)