gdbm: Resolve host contamination issue

The autoconf macros detect the presence of -ldbm or -lndbm on the host
system and add the library to link against, for now good reason I can
explain.

This patch makes the build behave determinstically whether they're
present or not. Other than the extra linkage, there doesn't appear to be
any other change in behaviour from these options and they look like
dead code.

The extra linkage can cause problems where sstate is used on a machine
where the extra librbary isn't present causing build failures.

(From OE-Core rev: f609bf5525450bfdb8e0864d44c41cce7f9319c9)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-support/gdbm/gdbm_1.10.bb

@@ -16,6 +16,9 @@ inherit autotools gettext lib_package
 # Needed for dbm python module
 EXTRA_OECONF = "-enable-libgdbm-compat"
 
+# Stop presence of dbm/nbdm on the host contaminating builds
+CACHED_CONFIGUREVARS += "ac_cv_lib_ndbm_main=no ac_cv_lib_dbm_main=no"
+
 BBCLASSEXTEND = "native nativesdk"
 
 do_install_append () {