Enabling opcache causes libraries to build as static

Hi all, I've been trying to get to the bottom of why even a minimal PHP build (any 5.x or 7 release, or a fresh e.g. 7.0 branch checkout after buildconf) creates .a versions of all shared extensions in addition to ..so modules. I've narrowed it down to opcache: $ ./configure --disable-all ... checking whether to build shared libraries... yes checking whether to build static libraries... no versus $ ./configure --disable-all --enable-opcache=shared ... checking whether to build shared libraries... yes checking whether to build static libraries... yes but I cannot figure out why this is happening (both on OS X and Ubuntu 14.04). Is this intentional? Related to opcache's "can only be built shared" status? But why would that trigger static builds? Or is it a bug? I've been wading through config.m4s, generated configures etc but still haven't found the reason; any pointers or explanations would be much appreciated! Cheers, David