Even though gnutls is a hard-req for libvirt, and gnutls depends
on libtasn1, that does not mean that you have to have the libtasn1
development files installed.  Skip the test rather than failing
compilation in that case.

With newer gcc, the test consumed too much stack space.  Move
things to static storage to fix that.

* configure.ac (AC_CHECK_HEADERS): Check for libtasn1.h.
(HAVE_LIBTASN1): New automake conditional.
* tests/Makefile.am (virnettlsconvirnettlscontexttest_SOURCES)
(virnettlscontexttest_LDADD): Allow compilation without libtasn1.
* tests/virnettlscontexttest.c: Skip test if headers not present.
(struct testTLSCertReq): Alter time members.
(testTLSGenerateCert): Reflect the change.
(mymain): Reduce stack usage.