In any major package for Linux, running ./configure --help will output at the end:

Some influential environment variables:
      CC          C compiler command
      CFLAGS      C compiler flags
      LDFLAGS     linker flags, e.g. -L<lib dir> if you have libraries in a
                  nonstandard directory <lib dir>
      CPPFLAGS    C/C++ preprocessor flags, e.g. -I<include dir> if you have
                  headers in a nonstandard directory <include dir>
      CPP         C preprocessor

Use these variables to override the choices made by `configure' or to help
it to find libraries and programs with nonstandard names/locations.

How do I use these variables to include a directory? I tried running ./configure --CFLAGS="-I/home/package/custom/" and ./configure CFLAGS="-I/home/package/custom/" however these do not work. Any suggestions?