libxml on Intel Mac

While compiling PHP 5 with libxslt on an Intel Mac Mini, I got these 
warnings:

/Users/elharo/Desktop/php-5.1.2/ext/libxml/libxml.c:264: warning: 
pointer targets in passing argument 1 of 'xmlParseURI' differ in signedness
/Users/elharo/Desktop/php-5.1.2/ext/libxml/libxml.c:265: warning: 
pointer targets in passing argument 1 of 'xmlStrncmp' differ in signedness
/Users/elharo/Desktop/php-5.1.2/ext/libxml/libxml.c:265: warning: 
pointer targets in passing argument 2 of 'xmlStrncmp' differ in signedness
/Users/elharo/Desktop/php-5.1.2/ext/libxml/libxml.c: In function 
'_php_list_set_error_structure':
/Users/elharo/Desktop/php-5.1.2/ext/libxml/libxml.c:433: warning: 
pointer targets in passing argument 1 of 'xmlStrdup' differ in signedness
/Users/elharo/Desktop/php-5.1.2/ext/libxml/libxml.c:433: warning: 
pointer targets in assignment differ in signedness

Anyone want to hazard a guess as to what's going on? libxml/libxslt is 
bundled with Mac OS X these days, so I suspect the problem is more 
likely to be in PHP than libxml, but I could be wrong about that.

How serious is this likely to be?

-- 
Elliotte Rusty Harold  
XML in a Nutshell 3rd Edition Just Published!
http://www.cafeconleche.org/books/xian3/
http://www.amazon.com/exec/obidos/ISBN=0596007647/cafeaulaitA/ref=nosim