Disable parallel Make execution (go-gitea#29186)

Ref:
https://www.gnu.org/software/make/manual/html_node/Parallel-Disable.html

> If the .NOTPARALLEL special target with no prerequisites is specified
anywhere then the entire instance of make will be run serially,
regardless of the parallel setting

Makefile

@@ -988,3 +988,8 @@ docker:
 
 # This endif closes the if at the top of the file
 endif
+
+# Disable parallel execution because it would break some targets that don't
+# specify exact dependencies like 'backend' which does currently not depend
+# on 'frontend' to enable Node.js-less builds from source tarballs.
+.NOTPARALLEL:

README.md

@@ -89,8 +89,6 @@ The `build` target is split into two sub-targets:
 
 Internet connectivity is required to download the go and npm modules. When building from the official source tarballs which include pre-built frontend files, the `frontend` target will not be triggered, making it possible to build without Node.js.
 
-Parallelism (`make -j <num>`) is not supported.
-
 More info: https://docs.gitea.com/installation/install-from-source
 
 ## Using