Gack. I totally missed this. (Sorry... it's late... or early rather...)

--cflags=-I/usr/local/mysql -libs=-L/path/to/mysql/libs

Should be

--cflags=-I/usr/local/mysql/include -libs=-L/usr/local/mysql/lib/mysql

That's why your compiler is unable to find mysql.h and errmsg.h. It doesn't have the right include directory set. (Assuming your mysql install directory is /usr/local/mysql/)