ERROR: libart-2.0 - Missing libart when installing RRDtool

configure: WARNING:
----------------------------------------------------------------------------
* I found a copy of pkgconfig, but there is no libart-2.0.pc file around.
  You may want to set the PKG_CONFIG_PATH variable to point to its
  location.
----------------------------------------------------------------------------

The issue is the RRDtool configure script is looking in /usr/include vs. /usr/local/include for the libart files.  So there are a few ways to tweak this.

1. You can symlink the files from where it is looking to where they are.  Can be very messy to get working, but you may unwittingly solve future issues where these files are needed.
2. You could change where you install libart to not use /usr/local

3. You can set the correct environment variable:
   export CPPFLAGS=' -I/usr/local/include/libart-2.0'

That solved my RRDTool installation issue.