build-sys: use the same modlibexecdir as PulseAudio

build-sys: use the same modlibexecdir as PulseAudio

When running ./configure, the old way of setting modlibexecdir
resulted in "/usr/local/lib/pulse-/modules" on my machine, which is
obviously wrong (it's missing the version number after "pulse-"). I
didn't track down the root cause (I suspect that the pkg-config
command used for getting the version didn't find the .pc file from
/usr/local), but I think the old way didn't make sense anyway. Why
would anyone want to use any other value for modlibexecdir than what
is used for building the upstream PulseAudio modules? This patch
reads the modlibexecdir value from pulseaudio-module-devel.pc. Now
the modlibexecdir value on my machine is
"/usr/local/lib/pulse-5.0/modules" as it should.

Change-Id: If8f9db70aea6c157d19f14ec370c571279fa5eb1