autoconf 2.69: Depends on dupes/m4 for Linuxbrew
autoconf 2.69: Depends on dupes/m4 for Linuxbrew

Change m4 to homebrew/dupes/m4.

Closes Homebrew/linuxbrew#24

LinuxbrewTestBot/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,forevergenin/homebrew-core,forevergenin/homebrew-core,rwhogg/homebrew-core,Linuxbrew/homebrew-core,maxim-belkin/homebrew-core