gmp: update to 6.1.2 for Linuxbrew
gmp: update to 6.1.2 for Linuxbrew

gmp: depends on m4 for Linuxbrew

Without this, building gmp from source failed on my Fedora machine.

Closes Linuxbrew/homebrew-core#4157.

Signed-off-by: Michka Popoff <7b0496f66f66ee22a38826c310c38b415671b832@gmail.com>

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