tests: fix bug in alloca*.test

tests: fix bug in alloca*.test

* tests/alloca.test: Make grepping of automake stderr stricter,
add a trailing `:' command; also, add AC_PROG_CC to configure.in,
and create a dummy alloca.c file, to ensure that we fail for the
proper reason.
* tests/alloca2.test: Likewise.  Also, look for LT_INIT, not
AC_PROG_LIBTOOL, in the error message (bug introduced with commit
v1.11-315-gd51e7b7 "libtool: suggest LT_INIT if LTLIBRARIES
primary is used").

From a report by Patrick Welche.