build: Use cmake's CMAKE_DLLTOOL instead of manually finding DLLTOOL.
build: Use cmake's CMAKE_DLLTOOL instead of manually finding DLLTOOL.

cmake 3.16 learned how to find the dlltool of the used toolchain.

jrfonseca/drmingw,jrfonseca/drmingw,jrfonseca/drmingw,jrfonseca/drmingw