C_INCLUDE_PATH and gcc -I option

[Madhur]

I would like to know the difference between using the
C_INCLUDE_PATH and using the -I option with compilers. How are they
different?

The problem which I am facing is that I am having a
source base which uses python. I am currently enabling compile time
option _POSIX_C_SOURCE in my Makefile. The compilaiton fails if i
include -I option to /usr/include/python2.4

/usr/include/python2.4/pyconfig-32.h:838:1: error: "_POSIX_C_SOURCE"
redefined

but if i do

export C_INCLUDE_PATH=/usr/include/python2.4

I do not face any compilation issues.

I would like to know if there is anything i am missing on this.