Add macOS instructions for installing mips-linux-gnu-gcc

docs/BUILDING_MACOS.md

@@ -60,7 +60,7 @@ Configure the build
 
 Make and install binutils
 ```bash
-make -j
+make -j$(nproc)
 sudo make install
 ```
 
@@ -82,3 +82,44 @@ If this worked, you can now delete the temporary directory `~/binutils-tmp`.
 Apple's version of `make` is very out-of-date, so you should use the brew-installed `gmake` in place of `make` in this repo from now on.
 
 You should now be able to continue from [step 2](../README.md#2-clone-the-repository) of the Linux instructions.
+
+
+## 4. Building GCC (optional)
+
+If you'd like to compile with GCC instead of IDO (e.g. for modding), you can build it from source similarly to how we built binutils:
+
+Install dependences
+```bash
+brew install gcc@14 gmp isl libmpc mpfr
+```
+
+Create and enter local working dir
+```bash
+mkdir ~/gcc-tmp
+cd ~/gcc-tmp
+```
+
+Get and extract gcc source
+```bash
+curl -O https://ftp.gnu.org/gnu/gcc/gcc-14.2.0/gcc-14.2.0.tar.xz
+tar xvf gcc-14.2.0.tar.xz
+```
+
+Create and enter a build directory
+```bash
+mkdir build-gcc
+cd build-gcc
+```
+
+Configure the build
+```bash
+CC=gcc-14 CXX=g++-14 ../gcc-14.2.0/configure --target=mips-linux-gnu --prefix=/opt/cross --disable-nls --enable-languages=c --with-gmp=$(brew --prefix)/opt/gmp --with-mpfr=$(brew --prefix)/opt/mpfr --with-mpc=$(brew --prefix)/opt/libmpc --with-isl=$(brew --prefix)/opt/isl
+```
+
+Make and install gcc
+```bash
+CC=gcc-14 CXX=g++-14 make all-gcc -j$(nproc)
+sudo make install-gcc
+```
+
+If this worked, you can now delete the temporary directory `~/gcc-tmp`.