for w64-based computers you have to install mingw64. If pkg-config.exe is missing then, you can refer to http://ftp.acc.umu.se/pub/gnome/binaries/win64/dependencies/
Unzip and copy/merge pkg-config.exe into your C:\mingw-w64 installation, eg. into on my pc into C:\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0\mingw64\bin