[mono] iOS Simulator builds will pick up a system ICU in some circumstances

[CoffeeFlux]

Error:

  ld: building for iOS Simulator, but linking in dylib built for macOS, file '/usr/local/opt/icu4c/lib/libicui18n.dylib' for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  ninja: build stopped: subcommand failed.

Env vars set (homebrew-recommended):

# icu4c is keg-only
export PATH="/usr/local/opt/icu4c/bin:$PATH"
export PATH="/usr/local/opt/icu4c/sbin:$PATH"
export LDFLAGS="-L/usr/local/opt/icu4c/lib $LDFLAGS"
export CPPFLAGS="-I/usr/local/opt/icu4c/include $CPPFLAGS"
export PKG_CONFIG_PATH="/usr/local/opt/icu4c/lib/pkgconfig:$PKG_CONFIG_PATH"

It should probably just never check the system, given I would not expect there to ever be a system install of iOS ICU builds.

[CoffeeFlux]

cc: @steveisok

[steveisok]

/cc @akoeplinger if it's still relevant.

[steveisok]

@akoeplinger Please check to see if this still happens. If not, close.