configure.ac: export an empty lib_prefix variable
this fixes the pkg-config files after:
*.pc.in: add lib prefix to lib names w/MSVC
@@ -749,6 +749,8 @@ fi
dnl =========================
+dnl Add an empty lib_prefix variable for use in *.pc.in.
+AC_SUBST([lib_prefix])
AC_CONFIG_MACRO_DIR([m4])
AC_CONFIG_HEADERS([src/webp/config.h])
AC_CONFIG_FILES([Makefile src/Makefile man/Makefile \
