use MakeMakefile.find_executable in place of MiniPortile#which

[flavorjones]

MiniPortile currently has a private methods, which, that searches the $PATH environment variable for an executable, and makes sure to do it safely on Windows as well as POSIX systems.

But Ruby's MakeMakefile module has a method find_executable that does the same thing. Let's use that instead and delete some code.

[stanhu]

find_executable emits a message to stdout (checking for X) every time it's called: https://github.com/ruby/ruby/blob/f302e725e10ae05e613e2c24cae0741f65f2db91/lib/mkmf.rb#L1637

Is that an issue?