> FWIW I build Emacs on MSYS2 and --without-imagemagick seems unnecessary.
> The configure script just says that Imagemagick is absent.

Indeed, I just built trunk and emacs-25 from inside mingw64.exe without specifying either --without-imagemagick or PKG_CONFIG_PATH, just with ./configure --prefix=`pwd`, and it went perfectly well.

> I have no idea about PKG_CONFIG_PATH. The MSYS2 recipe does not mention
> it and seems to work just fine.

Our instructions in nt/INSTALL.W64 do not talk about pkg-config, but they show PKG_CONFIG_PATH used with configure: