Support NMake build system for building generated code

[neworderofjamie]

I switched GeNN's Windows build system (both for building GeNN itself and generated code) from using NMake to MSBuild a long time ago as it's a nicer build system but, for building generated code, it has some disadvantages:

1. If you install the CUDA toolkit using Conda, the plugin required to build CUDA with MSBuild does not get installed
2. Neither ISPC or HIP provides MSBuild plugins and, with NMake, you can fall back on writing build rules by hand

Therefore, in this PR, I have resurrected support for building generated CUDA code using NMAKE if the MSBuild plugins are not available. I have also tidied up a couple of other Windows build related things:

1. Before GeNN 5, when using GeNN from C++, we linked against the generated code library which required a few awkward things which are no longer required as GeNN 5 always dynamically loads generated code:
   a. Decorating all the exported things with __declspec(dllexport) when we build the generated code and __declspec(dllimport) when we link against it.
   b. Windows libraries needed to be generated in the same directory as the script rather than in the _CODE directory which, in turn, also required them to have unique names. This is also no longer required and Windows now builds libraries called runner into the _CODE directory.
2. Another issue with Conda is that CUDA libraries don't live in $(CUDA_PATH)/x64/lib so we have added a separate CUDA_LIBRARY_PATH environment variable (that defaults to $(CUDA_PATH)/x64/lib) but can be overridden. I think this will also help with other weird environments.

[codecov]

Codecov Report

❌ Patch coverage is 1.61290% with 61 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.43%. Comparing base (2e66f81) to head (bae045d).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/genn/backends/cuda/backend.cc	0.00%	28 Missing ⚠️
src/genn/genn/code_generator/generateNMakefile.cc	0.00%	17 Missing ⚠️
src/genn/backends/single_threaded_cpu/backend.cc	0.00%	8 Missing ⚠️
pygenn/genn_model.py	0.00%	4 Missing ⚠️
src/genn/genn/code_generator/generateMSBuild.cc	0.00%	3 Missing ⚠️
...nclude/genn/backends/single_threaded_cpu/backend.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #700      +/-   ##
==========================================
- Coverage   88.83%   88.43%   -0.40%     
==========================================
  Files         106      107       +1     
  Lines       14749    14232     -517     
==========================================
- Hits        13102    12586     -516     
+ Misses       1647     1646       -1     

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[tnowotny]

I wish this could be a little less messy, but I appreciate the trade-off between the better build system (MSBuild) and the arguably more universal one (NMake) ... in terms of details, I need to trust you that this works.

[neworderofjamie]

It's a very annoying trade-off - I would just say to get rid of MSBuild for building generated code but NMake doesn't seem to track dependencies and that makes it way slower to build generated code