when cross-compiling, the host prefix is not added to "ar"
When building a static library (as in preprocessor/macro), the build system will always use "ar" (instead of "i686-w64-mingw32-ar" for example).
In most cases this is not a problem.
The problem can happen if one has only the cross-compiling binutils, but not the native binutils, as for example in a Cygwin install where only the packages listed on the wiki page are installed.