When compiling PHP from source (in my case version 5.3.2) on CentOS 5.4 I kept getting the following error when trying to compile with mcrypt:
"/usr/bin/ld: cannot find -lltdl"
This seems to be due to a missing development library with the obscure name of libtool-ltdl-devel to fix this simply use:
yum install libtool-ltdl-devel
Which should include the package along with several dependencies.