AC_CHECK_HEADER newbie problem

Hi I assume I installed myfile.h in $HOME/local/include/mylib I want to AC_CHECK_HEADER this file. In configure.in I do: CXXCPPFLAGS="$CXXCPPFLAGS -I$HOME/local/include" CPPFLAGS="$CPPFLAGS -I$HOME/local/include" AC_CHECK_HEADER(mylib/myfile.h, myfile=yes, myfile=no) AC_CHEC

include already set in CXXCPPFLAGS ?

Hi I submit this question for CXXCPPFLAGS, but it could be for any string. In a m4 macro file, is there a simple trick to detect if '-I/usr/local/include' is already set ? Thanks Genty Jean-Paul