Include some content when exporting abs_top_srcdir and builddir

FreeBSD make demands this.

src/test/include.am

 
 export PYTHON=@PYTHON@
 export SHELL=@SHELL@
-export abs_top_srcdir
-export builddir
+export abs_top_srcdir=@abs_top_srcdir@
+export builddir=@builddir@
 
 TESTSCRIPTS = src/test/test_zero_length_keys.sh