Recent -CURRENT unable to build numerous ports

Hi,

-CURRENT as of r307731 seems to have some serious flaw with its build
tool environment. Many ports fail to build with some error similar to
the following (devel/apr1):

cc -emit-llvm -fno-strict-aliasing -pipe -march=native -DLIBICONV_PLUG
  -fstack-protector -c -o .c.bco
cc: error: no input files
*** [.c.bco] Error code 1
[...]

Since not all ports are affected, I assume some auto[conf|tool|make]
issue to be triggered. Hints for getting this one solved are highly
appreciated.

Cheers
Marcus