relocate() in gnulib-lib needs INSTALLDIR and INSTALLPREFIX to be Windows paths so it can relocate the input. Fix by removing INSTALLPREFIX from MSYS2_ARG_CONV_EXCL so it gets converted when passed to the compiler and pass it to gnulib-lib, and convert INSTALLPREFIX in gettext-tools/config.h manually. Fixes #4392 (for real, hopefully)