The confINCDIRS macro defines the directories searched (using the compiler's -I switch) for #include files. In general, this will be empty unless you are using libraries that are not normally used. For example, you might have installed the db(3) library in /usr/local/lib and its corresponding include files in /usr/local/include/db. In this case, you would define:

APPENDDEF(`confINCDIRS', `-I/usr/local/include/db')
APPENDDEF(`confLIBDIRS', `-L/usr/local/lib')

Here, we use the APPENDDEF directive to prevent (possibly) prior values from being overwritten. The -I will be passed to the C compiler. The -L will be passed to the loader.

Note that the -I must appear as part of the value. If you omit that switch, Build will not correct the mistake and your build of sendmail will fail.