Mark top-level Makefile .NOTPARALLEL to fix make -j build race

[darrylabbate]

The IMB-* targets each recurse into src_cpp/ and compile a shared set of intermediate objects (imb.o, benchmark_suites_collection.o, args_parser.o, scope.o, libyaml-cpp.a) to identical paths. Building two or more suites concurrently under 'make -j' lets multiple sub-makes write those same object files simultaneously, clobbering them and intermittently breaking the link, e.g.:

benchmark_suites_collection.h: undefined reference to
  `BenchmarkSuitesCollection::pnames'
imb.o: file not recognized: File truncated

The suite recipes invoke a plain 'make' (not $(MAKE) and not prefixed with '+'), so they are not recognized as recursive. With GNU Make < 4.4's pipe-based jobserver such sub-makes were cut off from the job pool and ran serially, which masked the race. GNU Make >= 4.4's FIFO jobserver connects non-recursive sub-makes to the parallel pool, so the suite builds now run concurrently and the race surfaces routinely (observed on Ubuntu 26.04 / RHEL 10, which ship make 4.4).

Add .NOTPARALLEL so the top-level suite builds are serialized regardless of -j. Parallelism within each suite sub-make is unaffected, and the suites build in seconds, so the wall-clock impact is negligible.

[darrylabbate]

Refactoring s.t. the Makefile emits per-suite objects is probably the better long-term solution (allows parallelizing), but declaring .NOTPARALLEL effectively reverts it to its previous behavior with Make 4.4+.