nitroImplSrcVar function
The CMake CACHE variable name a Windows/Linux caller sets (before
add_subdirectory("../src")) to override which impl file gets compiled
for target — see _implSourcesBlock and _linkDesktopCMake /
needsSeparateWindowsImpl / needsSeparateLinuxImpl in link_command.dart.
Keyed per-target (not one global name) so multi-spec plugins with several
C++ module libraries in one src/CMakeLists.txt can override each
independently.
Implementation
String nitroImplSrcVar(String target) => 'NITRO_IMPL_SRC_$target';