bug#33683: sorcer fails to build

[Thorsten Wilms]

phase `unpack' succeeded after 0.0 seconds
starting phase `build-faust-sources'
ERROR : can't open architecture file lv2synth.cpp
phase `build-faust-sources' failed after 0.0 seconds
## WARNING: phase `build-faust-sources' returned `#f'.  Return values 
other than #t
## are deprecated.  Please migrate this package so that its phase
## procedures report errors by raising an exception, and otherwise
## always return #t.
builder for 
`/gnu/store/kkv8im08v7x5ijd8w4jgmgvcp42a2i8l-sorcer-1.1.3.drv' failed 
with exit code 1
build of /gnu/store/kkv8im08v7x5ijd8w4jgmgvcp42a2i8l-sorcer-1.1.3.drv failed
View build log at 
'/var/log/guix/drvs/kk/v8im08v7x5ijd8w4jgmgvcp42a2i8l-sorcer-1.1.3.drv.bz2'.
guix build: error: build failed: build of 
`/gnu/store/kkv8im08v7x5ijd8w4jgmgvcp42a2i8l-sorcer-1.1.3.drv' failed

---

I found this thread, but that's about the warning, not the actual cause 
of failure, AFAICS:
http://lists.gnu.org/archive/html/bug-guix/2018-06/msg00277.html

Thanks for handling the previous issues so quickly!