Unable to install XML package in R on CentOS

You have two problems clearly stated in the output:

1. The first is a warning, you may want to remove a.out/:

checking for C compiler default output file name... 
rm: cannot remove 'a.out.dSYM': Is a directory
a.out
checking whether the C compiler works... yes

2. The second is an error and your dealbreaker -- you do not have libxml2-dev (or whatever CentOS calls this) installed:

checking for xml2-config... no
Cannot find xml2-config

Without the XML development tools provided by libxml2-dev you simply cannot build the package from source.