* configure.in: Test for libintl.h.

[Loren James Rittle]

Phil, is it possible that this patch broke rebuild/bootstrap of
libstdc++-v3 on a port with:

i386-unknown-freebsd4.8/libstdc++-v3/src/.libs/libstdc++.so: 
undefined reference to `libintl_gettext'

while building abi_check? and then every link test during a make check?

I have /usr/local/include/libintl.h on my system but no
/usr/include/libintl.h .  Likewise, support for that header is not in
libc since I installed GNU libintl.  If you want to use stuff declared
out of <libintl.h>, you have to link -lintl using one of the various
techniques.

I have confirmed that if I merely add: -lintl to the end of the
command line, then it works as expected.

Regards,
Loren