I was working on building the mysql ruby gem. My mysql install is in a non-standard location. I could get the gem to build with syntax like:

gem install mysql -- --with-mysql-config=/path/to/bin/mysql_config

However, I noticed that the resulting library did not embed the path to the mysql library, ldd on the mysql.so indicated:

libmysqlclient.so.16 => not found

I did not want to have to fiddle with setting an LD_LIBRARY_PATH at runtime. The way to address this is to pass -R or -rpath to the linker at compile time to embed the path to the library. The mysql gem uses the standard extconf.rb to build a Makefile to build the library. extconf.rb uses the standard Ruby mkmf library to do the heavy lifting. Thanks to the woeful lack of documentation for mkmf it took me several hours to figure out how to convince mkmf to include the proper option in the compiler flags. The answer is:

gem install mysql -- --with-mysql-config=/path/to/bin/mysql_config --with-dldflags="-Wl,-rpath,/path/to/lib/mysql"

--with-dldflags appends anything you specify to the DLDFLAGS variable in the generated Makefile. FYI, --with-ldflags overwrites DLDFLAGS with what you specify, possibly wiping out other linker flags you want to keep. Also FYI, the -Wl,-rpath,etc. syntax is required to pass the linker option through the compiler.

ldd now shows that the path is embeded in the resulting mysql.so:

libmysqlclient.so.16 => /path/to/lib/mysql/libmysqlclient.so.16 (0x00002ad5e81d1000)