make jobserver integration
==========================

It seems to be possible with both GNU Make and pmake/BSD Make.

GNU Make
--------
Modify command to start with +:

    myapp: main.slul
        +cslul $(srcdir)/main.slul

NOTE!!! GNU Make 4.4 will change how the jobserver works from a pipe to mkfifo.

pmake/BSD Make
--------------
BSD Make seems to create the file descriptors if the recipe contains $(MAKE)
somewhere (even in arguments to commands). The file descriptors have high
numbers such as 15,16.

    myapp: main.slul
        cslul --ignore $(MAKE) $(srcdir)/main.slul


Combined solution
-----------------

    myapp: main.slul
        +cslul $(srcdir) --outdir . --jobserver $(MAKE)


Checking MAKEFLAGS
------------------
Jobserver FD detection:

    GNU Make:
    --jobserver-auth=3,4
    BSD Make:
    -J 15,16

Detecting -n option (don't run commands):

    GNU Make:
    n    (first word is n)
    BSD Make:
    -n   (contains option -n)

Fortunately, BSD make does not add the targets in MAKEFLAGS, so
if there is a target called "n", it will not show up.

Additionally, -t might need to be detected (touch targets):

    GNU and BSD Make:
    -t   (contains option -t)


Supporting pthreads
-------------------
Threading (or forking) is necessary to support
parallel compilation of a single module.

Perhaps something like this in the cslul Makefile:

    THREADFLAGS = $(THREADS:pthread=-pthread -DENABLE_PTHREAD)
    THREADFLAGS = $(THREADS:winapi=-DENABLE_WINAPI_THREADS)

    .c.o:
        $(CC) ... $(THREADFLAGS) $(CFLAGS)

    cslul:
        $(CC) ... $(THREADFLAGS) $(CFLAGS)

Then, in the source:

    #if defined(ENABLE_PTHREAD)
        ...
    #elif defined(ENABLE_WINAPI_THREADS)
        ...
    #else
        ...
    #endif

Makefile invocation:

    make THREADS=pthread cslul