-isystem${prefix}/include instead of -I${prefix}/include

[Ryan Schmidt]

Many ports now replace the -I${prefix}/include that MacPorts  
automatically puts into the CPPFLAGS with -isystem${prefix}/include  
to work around issues. (freetype ghostscript jpeg pdflib qt4-kde qt4- 
mac-devel qt4-mac qt4-x11 texlive_base)

Should we make this change in MacPorts base?

When I brought this situation to the attention of the developer of  
freetype, he said that what we are doing with -I is "completely non- 
standard", and that using -isystem would be "exactly the right thing."

http://lists.gnu.org/archive/html/freetype/2009-04/msg00005.html


What about LDFLAGS? We currently do -L${prefix}/lib. Is this also non- 
standard and problematic? (Could it be the cause of #18937?) Should  
we do something else? I thought someone mentioned something about  
this but I can't find it now and can't figure out how to search for  
it in the archives.


I've always wondered how C_INCLUDE_PATH and LIBRARY_PATH compared to  
what we do, but if we find a solution in -isystem and whatever its  
library analog is, then I don't care.