[devdocs] Improve documentation about building external forks of LLVM

Make.inc

@@ -357,6 +357,10 @@ USE_MLIR := 0
 # Options to use RegionVectorizer
 USE_RV := 0
 
+# Use `ccache` for speeding up recompilation of the C/C++ part of Julia.
+# Requires the `ccache` executable to be in the `PATH` environment variable.
+USECCACHE := 0
+
 # Cross-compile
 #XC_HOST := i686-w64-mingw32
 #XC_HOST := x86_64-w64-mingw32

doc/src/devdocs/build/build.md

@@ -60,6 +60,8 @@ To run julia from anywhere you can:
 
 - write `prefix=/path/to/install/folder` into `Make.user` and then run `make install`. If there is a version of Julia already installed in this folder, you should delete it before running `make install`.
 
+Some of the options you can set to control the build of Julia are listed and documented at the beginning of the file `Make.inc`, but you should never edit it for this purpose, use `Make.user` instead.
+
 Now you should be able to run Julia like this:
 
     julia
@@ -239,10 +241,32 @@ For packaging Julia with LLVM, we recommend either:
  - bundling a Julia-only LLVM library inside the Julia package, or
  - adding the patches to the LLVM package of the distribution.
    * A complete list of patches is available in on [Github](https://github.com/JuliaLang/llvm-project) see the `julia-release/15.x` branch.
-   * The only Julia-specific patch is the lib renaming (`llvm-symver-jlprefix.patch`), which should _not_ be applied to a system LLVM.
+   * The only Julia-specific patch is the lib renaming (`llvm7-symver-jlprefix.patch`), which should _not_ be applied to a system LLVM.
    * The remaining patches are all upstream bug fixes, and have been contributed into upstream LLVM.
 
-Using an unpatched or different version of LLVM will result in errors and/or poor performance. Though Julia can be built with newer LLVM versions, support for this should be regarded as experimental and not suitable for packaging.
+Using an unpatched or different version of LLVM will result in errors and/or poor performance.
+You can build a different version of LLVM from a remote Git repository with the following options in the `Make.user` file:
+
+```make
+# Force source build of LLVM
+USE_BINARYBUILDER_LLVM = 0
+# Use Git for fetching LLVM source code
+DEPS_GIT = 1
+# URL of the Git repository you want to obtain LLVM from:
+LLVM_GIT_URL = ...
+# Name of the branch to check out automatically
+LLVM_BRANCH = standalone_rel_15_x
+LLVM_SHA1 = $(LLVM_BRANCH)
+
+# Other useful options:
+# List of LLVM targets to build.  It is strongly recommended to keep at least all the
+# default targets listed in `deps/llvm.mk`, even if you don't necessarily need all of them.
+# LLVM_TARGETS = ...
+# Use ccache for faster recompilation in case you need to restart a build.
+# USECCACHE = 1
+```
+
+Though Julia can be built with newer LLVM versions, support for this should be regarded as experimental and not suitable for packaging.
 
 ### libuv