The PATH environment variable is unrelated to the compiler/linker finding libraries...

But as you're saying you have the lib installed in /usr/lib (which is a default library search path), I suppose the problem is that the installation somehow didn't succeed...  Did you actually run ./configure --prefix=/usr ; make install (the default installation prefix would be /usr/local) from the directory where you unpacked the libxml2 sources (the tarball you downloaded), and did the build and installation complete without errors?