Build: let a parent make control the wxWidgets build jobs

[flatstik]

The wxbuild target always runs the wxWidgets sub-make with -j 4. When VeraCrypt is built from a parent make -jN, as distribution or embedded build systems (e.g. OpenWrt) do, the explicit -j 4 overrides the parent's job count and jobserver: the wx build ignores -j1 or the configured parallelism and can oversubscribe the machine.

This keeps -j 4 as the default for a standalone make, but leaves the job count to the parent when it was started with -j. The parent's MFLAGS then contains -j, -jN or the jobserver option, so the sub-make joins the parent's jobserver. MFLAGS is used rather than MAKEFLAGS because it holds only options, so a command-line variable whose value contains -j (e.g. make 'FOO=a -j3') can't match.

Single-letter options can be grouped in one MFLAGS word (GNU make 3.81 gives -sj for the -s this Makefile adds plus a bare -j), so the check looks for j in every single-dash option word, skipping the options that take an argument (-C, -E, -I, -O, -W, -f, -o). Known limitation, documented next to the variable: an option argument containing a space followed by -j (e.g. -I '/tmp/ordinary -j3') still looks like a job option, so that build inherits the parent's (serial) setting instead of using 4 jobs; WX_MAKE_JOBS can be set explicitly in that case.

The OpenWrt recipe in src/Build/Packaging/openwrt now passes $(PKG_JOBS) and an empty WX_MAKE_JOBS to wxbuild: OpenWrt clears MAKEFLAGS and passes the job count in PKG_JOBS, which can be empty for a serial build, so the empty WX_MAKE_JOBS makes the wxWidgets build follow OpenWrt's settings in every case.

Checked with GNU make 4.4.1:

invocation	wx sub-make gets
make	-j 4 (as before)
make -j8	the parent's -j8 and --jobserver-auth=...
make -j	the parent's unlimited -j
make -j1	the parent's -j1 (serial; GNU make 4.2+, older makes do not record -j1 and give -j 4)
make -k, make -ks	-j 4
make 'FOO=a -j3'	-j 4
MFLAGS=-sj (GNU make 3.81, bare -j)	the parent's unlimited -j
make -I/jail, -C/proj, -Otarget	-j 4

The grouped MFLAGS forms were checked by evaluating the same expression with the MFLAGS values make 3.81 produces.

See also #717, which proposed dropping -j 4 for the same reason (closed by its author, not rejected). This version keeps the standalone default, so plain make builds aren't slowed down.

[flatstik]

@idrassi any comments on this? The OpenWrt build is still waiting for your decision for this

[idrassi]

Thanks for the update. Looks good to me now.