test-memmem takes waaay too long

[Ralf Wildenhues]

Hello, and a Happy New Year,

quoting memmem.m4:

    AC_CACHE_CHECK([whether memmem works], [gl_cv_func_memmem_works],
      [AC_RUN_IFELSE([AC_LANG_PROGRAM([#include <string.h>],
          [return !memmem ("a", 1, NULL, 0);])],
        [gl_cv_func_memmem_works=yes], [gl_cv_func_memmem_works=no],
        [dnl pessimistically assume the worst, since even glibc 2.6.1
         dnl has quadratic complexity in its memmem
         gl_cv_func_memmem_works=no])])

This sets gl_cv_func_memmem_works=yes in the non-cross-compiling case,
tested on GNU/Linux with glibc 2.3.6, which in turn causes the
test-memmem.c test to take an awful lot of time to complete (I killed
it after several minutes).  I noticed this with git master M4.

I suggest that either the testsuite test use some time limit (alarm?),
or the configure test tries to expose the quadratic complexity, or the
configure test assume `no' on systems where we know libc's memmem to be
insufficient.

Cheers,
Ralf