finally things were not so difficult. the javascript script create a file config.mingw that is easy to edit, in this case manually adding the paths separately with -I, in my case:
INCLUDE+= -IC:\cpp\libraries\iconv\GnuWin32\include -IC:\cpp\libraries\zlib1.2.3\GnuWin32\include
LIB+= -LC:\cpp\libraries\iconv\GnuWin32\lib -LC:\cpp\libraries\zlib1.2.3\GnuWin32\lib