A long time after...
I encounter the same trouble.
I don't know if debian doesn't create the good headers or if denora doesn't detect the libmysqlclient-dev, gonna have a look.
I don't really know what denora is expecting in the mysql_config to determine the presence of mysqlclient-dev
$ mysql_config --help
Usage: /usr/bin/mysql_config [OPTIONS]
Options:
--cflags [-I/usr/include/mysql -DBIG_JOINS=1 -fno-strict-aliasing -g]
--include [-I/usr/include/mysql]
--libs [-L/usr/lib/x86_64-linux-gnu -lmysqlclient -lpthread -lz -lm -lrt -ldl]
--libs_r [-L/usr/lib/x86_64-linux-gnu -lmysqlclient_r -lpthread -lz -lm -lrt -ldl]
--plugindir [/usr/lib/mysql/plugin]
--socket [/var/run/mysqld/mysqld.sock]
--port [0]
--version [5.5.37]
--libmysqld-libs [-L/usr/lib/x86_64-linux-gnu -lmysqld -lpthread -lz -lm -lrt -lwrap -lcrypt -ldl -laio]
--variable=VAR VAR is one of:
pkgincludedir [/usr/include/mysql]
pkglibdir [/usr/lib/x86_64-linux-gnu]
plugindir [/usr/lib/mysql/plugin]