mail/thunderbird: I finally found out the cause of its massive memory requirement for building

[PHO <pho%cielonegro.org@localhost>]

Hi,

I just committed a change to mail/thunderbird/options.mk, which 
tremendously reduces its RAM usage at build time from >32 GiB to ~3 GiB.

Thunderbird has been known for requiring insane amount of memory for 
building, but it got far worse as of 159.0. While cargo was building 
gkrust it consumed all of my 16 GiB RAM allocated for my NetBSD VM, and 
also ate up my entire 16 GiB swap space. So I re-run the make and 
checked `ps auxww|grep rustc` to see what exactly was happening, and I 
saw a dreadful "-C debuginfo=2" in its argv.

What the f**k. debuginfo=2?! I never asked for a debug build, yet it 
tried to generate debug symbols which will be thrown away at 
installation time.

Turned out there was a separate option for disabling debug symbols, 
namely --disable-debug-symbols, which had to be explicitly given if one 
wanted to stop it from getting killed by the kernel for running short of 
swap. Mozilla forks appear to think it's okay to consume more than 32 
GiB of RAM by default, even when a non-debug build is requested via 
--disable-debug. I firmly believe it's a bug, but meh...

As a bonus the disk consumption is also reduced now. Just after make 
succeeds the size of WRKDIR is now "only" 9.8 GiB. Can you believe it? 
It was like 40 GiB in the past.