LLVM port(s) take very long time to package

[Alexey Dokuchaev]

Hi there porters,

It had been annoying me for a while, so I figured I try asking here:
LLVM ports (particularly, llvm39-3.9.1) take abnormally long time to
package, e.g. right now it took 43 minutes (tmpfs-backed build area,
pkg-1.10.0_2.txz, AMD A8 APU @1900 MHz, plenty of RAM, no swap).

pkg was eating 100% (of a single core) during that time, so it looks
like it's actually busy (not waiting for I/O or something), but with
what?  And can it be fixed?  LLVM ports are real PITA to (re)build,
and while ccache helps to greatly reduce the build times, 3/4-hour
package time still cripples it.

./danfe