FindPackageModeMakefileTest: macOS needs isysroot flag

macOS/Xcode no longer populate /usr/include. Building with the compiler directly instead of using /usr/bin/c++ can result in missing headers without the -isysroot flag.

This technically fixes the immediate test failure reported in #19885, but that issue also mentions subsequent linker warnings that will probably become hard errors in a future macOS/Xcode version.