Release switching from header-only to proper .so/.a library to avoid
triggering failure with gnulib.

See: https://bugs.gentoo.org/946390

Haelwenn (lanodan) Monnier (5):
      Revert "merge error.c into error.h"
      error.h: remove deprecation warning
      Add pkg-config, static & shared targets
      Makefile,error-standalone.pc.in: support configurable INCDIR
      Makefile: set soname to liberror.so