gcc7 is needed instead of gcc49

Looking at `homebrew-gcc_cross_compilers` at redox-os/homebrew-gcc_cross_compilers!4 (062060c4), 
it looks like gcc7 is needed and not gcc49.

src/getting_started/preparing_the_build.md

@@ -42,13 +42,13 @@ $ [your package manager] install cmake make nasm qemu pkg-config libfuse-dev wge
 MacOS Users using MacPorts:
 
 ```
-$ sudo port install make nasm qemu gcc49 pkg-config osxfuse x86_64-elf-gcc
+$ sudo port install make nasm qemu gcc7 pkg-config osxfuse x86_64-elf-gcc
 ```
 
 MacOS Users using Homebrew:
 
 ```
-$ brew install make nasm qemu gcc49 pkg-config Caskroom/cask/osxfuse
+$ brew install make nasm qemu gcc@7 pkg-config Caskroom/cask/osxfuse
 $ brew install redox-os/gcc_cross_compilers/x86_64-elf-gcc
 ```