build gdbm with '--enable-libgdbm-compat' to generate libgdbm_compat.so.

component/gdbm/buildout.cfg

@@ -9,6 +9,7 @@ url = ftp://ftp.gnu.org/gnu/gdbm/gdbm-${:version}.tar.gz
 md5sum = 88770493c2559dc80b561293e39d3570
 configure-options =
   --disable-static
+  --enable-libgdbm-compat
 # install as parts/gdbm/include/gdbm/*.h etc. because some softwares
 # (eg. python's dbmmodule.c extension) assume the location like this.
 includedir = ${buildout:parts-directory}/${:_buildout_section_name_}/include