master does not compile on Nim 0.18.0
Steps to reproduce:
 * Checkout Nim 0.18.0 as per instructions in nim-lang/Nim
 * `koch nimble`

Expected result:
 * nimble compiles

Actual result:
 * Compilation fails with error message:
   `nimblepkg/nimscriptsupport.nim(48, 6) Error: undeclared identifier: 'NimCompilerApiVersion'`

This seems to be related to https://github.com/nim-lang/nimble/commit/e6c41248e62802b782cd740d309f951c6cb8eadd#diff-0b807079b25d2ca36e9badf9a2f43b53R48 not guarding against an undefined NimCompilerApiVersion.