Avoid using libtool wrapper for makekeys

For Windows targets, libtool uses a wrapper executable, not a wrapper
script (see [1]), which it compiles with the host compiler.  This
doesn't work when cross-compiling.

Since we don't actually need to link with anything, use the libtool flag
-all-static to tell it to stay completely out of this.

[1] https://www.gnu.org/software/libtool/manual/html_node/Wrapper-executables.html