I'm trying to build mosh from source on a SUSE10 machine and am getting the following error:

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

No package 'protobuf' found

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

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

I downloaded the source to protobuf and installed it in a custom path as well. I'm not using a package manager for any of this and cannot for various reasons outside the scope of the question. I added that custom path to my PATH and rehashed. Typically, this is enough for configure but in this case its not doing the trick. I added the prefix for protobuf to PKG_CONFIG_PATH but am still hitting this error.

What should I do next to get past this error?