botan-1.10.pc contains: prefix=/usr/local exec_prefix=${prefix} libdir=${prefix}/lib includedir=${prefix}/include/botan-1.10 but the headers are installed in /usr/include, not /usr/local/include.
Patch fixing the issue posted at http://patchwork.ozlabs.org/patch/460150/.
Fixed by: commit 7536a7354c92e0ec2760479a9d62e792703154d7 Author: Thomas Petazzoni <thomas.petazzoni@free-electrons.com> Date: Fri Apr 10 17:55:23 2015 +0200 botan: pass --prefix=/usr to fix .pc file Thanks.