CMake helper function 'find_link_library' is broken.

[WildCard65]

The current implementation of the custom helper function 'find_link_library' is

function(find_link_library libname outlibname)
  check_linker_flag(C "-l${libname}" mi_has_lib${libname})
  if (mi_has_lib${libname})
    message(VERBOSE "link library: -l${libname}")
    set(${outlibname} ${libname} PARENT_SCOPE)
  else()
    find_library(MI_LIBPATH libname)
    if (MI_LIBPATH)
      message(VERBOSE "link library ${libname} at ${MI_LIBPATH}")
      set(${outlibname} ${MI_LIBPATH} PARENT_SCOPE)
    else()
      message(VERBOSE "link library not found: ${libname}")
      set(${outlibname} "" PARENT_SCOPE)
    endif()
  endif()
endfunction()

As it currently stands, it is 100% broken in the event the linker flag check fails, this is because it passes "libname" instead of "${libname}" to find_library AND will silently abort if MI_LIBPATH is set to a library path already (in a previous call).

To fix this, obviously "libname" -> "${libname}" is required, but one of two solutions can be deployed for the variable situation:

1. Explicitly set "MI_LIBPATH" to the value "MI_LIBPATH-NOTFOUND" to force find_library to run, this can be a normal variable.
2. Do what the linker flag check does and append "${libname}" to the test variable: "MI_LIBPATH" -> "MI_LIBPATH${libname}".

Markdown is hiding underscores in the suggested fixes for the MI_LIBPATH variable.