libunistring.m4 macro does not support --with-libunistring

[Christian Grothoff]

We're using the libunistring.m4 macro from gnulib to test for 
libunistring.

However, I get user complaining that it doesn't support specifying an 
install path for the library:

https://gnunet.org/bugs/view.php?id=2060

The reporter writes:
>>
My command line: ./configure --prefix=/home/gnunet 
--with-extractor=/home/gnunet --with-libmicrohttpd=/home/gnunet 
--with-libunistring-prefix=/usr/local
The error in config.log:
configure:22576: checking for libunistring
configure:22595: gcc -o conftest -fno-strict-aliasing -Wall -g -O2 
-I/home/gnunet/include -L/home/gnunet/lib conftest.c -lm -ldl 
-lunistring >&5
conftest.c:83:21: error: uniconv.h: No such file or directory
conftest.c: In function 'main':
conftest.c:87: warning: implicit declaration of function 
'u8_strconv_from_locale'
configure:22595: $? = 1

As you can see there is no -I/usr/local/include -L/usr/local/lib like 
you would expect.
<<

The macro is from:
http://www.gnu.org/software/gnulib/MODULES.html#module=libunistring

I'd appreciate a fix ;-).


Happy hacking,

Christian