Gentoo's Bugzilla –
Attachment 553822 Details for
Bug 670094media-libs/mesa-18.* - ld: src/gallium/auxiliary/libgallium.a(driver_ddebug_dd_context.c.o): in function `u_thread_create.constprop.1': dd_context.c:(.text+0x170f): undefined reference to `pthread_sigmask'