Compilation with sparsehash fails

Environment is RHEL 6.3, GCC 4.7.2 (from devtools-1.1), sparsehash-devel 1.7 installed, graph-tool 2.2.28:

checking for SPARSEHASH... no configure: error: Package requirements (libsparsehash) were not met:

No package 'libsparsehash' found

Consider adjusting the PKG_CONFIG_PATH environment variable if you installed software in a non-standard prefix.

I am wondering if this is due to the location /usr/local/include/google/sparsehash instead of just /usr/local/include/sparsehash, or the fact that it's a header-only library?

Alternatively, you may set the environment variables SPARSEHASH_CFLAGS and SPARSEHASH_LIBS to avoid the need to call pkg-config. See the pkg-config man page for more details.