Compilation trouble on macOS

[rsmmr]

There's a file naming conflict: system headers contain #include <version>, which can end up picking up our VERSION file on the Mac's case-insensitive file system.

I've started seeing that only recently, not sure what changed; but now I'm actually also getting it when doing zkg install spicy-plugin:

In file included from /Users/robin/.zkg/testing/spicy-plugin/clones/spicy-plugin/compiler/driver.cc:3:
In file included from /Users/robin/.zkg/testing/spicy-plugin/clones/spicy-plugin/compiler/driver.h:5:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk/usr/include/c++/v1/map:481:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk/usr/include/c++/v1/__tree:14:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk/usr/include/c++/v1/iterator:419:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk/usr/include/c++/v1/__functional_base:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk/usr/include/c++/v1/type_traits:417:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk/usr/include/c++/v1/cstddef:37:
/Users/robin/.zkg/testing/spicy-plugin/clones/spicy-plugin/version:1:1: error: expected unqualified-id
0.99.4

[rsmmr]

(I did upgrade to 11.3, but this started already before that)