autoconf: Add link tests to each AC_CHECK_FUNC() test

autoconf: Add link tests to each AC_CHECK_FUNC() test

Update configure.ac to test libraries for getaddrinfo, strcasestr, memmem,
strlcpy, strtoumax, setenv, unsetenv, and mkdtemp.  The default compilers
on FreeBSD 4.9-SECURITY and FreeBSD 6.2-RELEASE-p4 do not generate warnings
for missing prototypes unless `-Wall' is used. This behavior renders the
results of AC_CHECK_FUNC() void on these platforms. The test AC_SEARCH_LIBS()
verifies a function is valid by linking to symbol within the system libraries.

Since this pattern needs to be repeated for many functions that are
checked with AC_CHECK_FUNC(), we add GIT_CHECK_FUNC() to drive the two
autoconf macros together.

Signed-off-by: David M. Syzdek <david.syzdek@acsalaska.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>