I recently tried to install the devtools R package (v1.8) on my Mac that was running OS X Yosemite (v10.10.4 - 14E46) with Xcode (v6.4 - 6E35b). My R (v3.2.2) was installed using homebrew.
I ran the following command in my R console:
Only to be met with the following errors:
./xml2_utils.h:5:10: fatal error: 'libxml/tree.h' file not found #include <libxml/tree.h>
The error is actually an issue with the xml2 R package which is dependency of devtools. I made a separate post, Installing the xml2 R Package - “fatal error: ‘libxml/tree.h’” that solves this problem.