Add `.NOTPARALLEL` directive to Makefile

[MisterDA]

If make is invoked with its -j option, given through the command line or with the MAKEFLAGS environment variable, it will build the recipes in parallel. The cmdliner build system doesn't support parallel execution and is prone to races (for instance, there's a TOCTOU in mkdir).

The .NOTPARALLEL directive of Make disables parallel execution. The GNU Make version shipped in macOS doesn't support .WAIT, so for compatibility rely on this all-in-one variable.

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

Note that the -j option of Make isn't used in the opam file, where -j%{jobs} could have been passed.

$ make all -j4
ocaml build.ml cma
ocaml build.ml cmxa
ocaml build.ml cmxs
mkdir: cannot create directory ‘C:\\Users\\misterda\\Tarides\\cmdliner\\_build’: File exists
exited with 1: "mkdir" "C:\Users\misterda\Tarides\cmdliner\_build"
mkdir: cannot create directory ‘C:\\Users\\misterda\\Tarides\\cmdliner\\_build’: File exists
make: *** [Makefile:63: build-native] Error 1
exited with 1: "mkdir" "C:\Users\misterda\Tarides\cmdliner\_build"
make: *** Waiting for unfinished jobs....
make: *** [Makefile:66: build-native-dynlink] Error 1