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

(cherry picked from commit 69ed1a4)

Conflicts:
	README.md

Makefile

@@ -1023,3 +1023,8 @@ 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: