Merge pull request #9635 from 9999years/document-makefile-variables

Document more `Makefile` variables

doc/manual/src/contributing/hacking.md

@@ -66,13 +66,24 @@ You can also build Nix for one of the [supported platforms](#platforms).
 
 ## Makefile variables
 
+You may need `profiledir=$out/etc/profile.d` and `sysconfdir=$out/etc` to run
+`make install`.
+
+You may want to set `MAKEFLAGS="-e -j $NIX_BUILD_CORES"` to allow environment
+variables to override `Makefile` variables.
+
 - `ENABLE_BUILD=yes` to enable building the C++ code.
 - `ENABLE_DOC_GEN=yes` to enable building the documentation (manual, man pages, etc.).
+
+  The docs can take a while to build, so you may want to disable this for local development.
 - `ENABLE_FUNCTIONAL_TESTS=yes` to enable building the functional tests.
 - `ENABLE_UNIT_TESTS=yes` to enable building the unit tests.
 - `OPTIMIZE=1` to enable optimizations.
-
-  The docs can take a while to build, so you may want to disable this for local development.
+- `libraries=libutil programs=` to only build a specific library (this will
+  fail in the linking phase if you don't have the other libraries built, but is
+  useful for checking types).
+- `libraries= programs=nix` to only build a specific program (this will not, in
+  general, work, because the programs need the libraries).
 
 ## Building Nix