Add documentation on JSC_UNIFIED_BUILD

Author: facekapow

src/build-instructions.md

@@ -156,6 +156,8 @@ If you want to build just the 64bit components, use `-DTARGET_i386=off` to disab
 
 Another way to speed up the build is to run `make` with multiple jobs. For this, run `make -j8` instead, where 8 is a number of current jobs to run of your choosing. In general, avoid running more jobs than twice the amount CPU cores of your machine.
 
+If you still want to build JavaScriptCore and have a bit of RAM to spare, JavaScriptCore also supports a build mode known as ["unified builds"](https://blogs.gnome.org/mcatanzaro/2018/02/17/on-compiling-webkit-now-twice-as-fast/). This build mode can cut JSC build times in half, at the expense of causing slightly higher RAM usage. This build mode can be enabled in Darling by adding `-DJSC_UNIFIED_BUILD=ON` when configuring the build.
+
 If you run LLDB and encounter messages indicating a lack of debug symbols, make sure you are doing a debug build. To do this, use the `-DCMAKE_BUILD_TYPE=Debug`.
 
 Darling tries to stick to a standard macOS installation as much as possible. However, if you would like to build and install some additional packages (such as GNU tar), you can add `-DADDITIONAL_PACKAGES=ON`.