Why is $prefix/include not inside cpp searchpath?

[Per Bothner]

Martin Kahlert wrote:

>At least gcc-3.0.* search inside $prefix/include:
>$ gcc -v -c c.c
>
>Reading specs from /sw/snapshots/lib/gcc-lib/i686-pc-linux-gnu/3.0.2/specs
>Configured with: ../gcc-20010917/configure --prefix=/sw/snapshots --enable-languages=c++,f77,java
>
>...
>#include <...> search starts here:
> /sw/snapshots/include
> /sw/snapshots/lib/gcc-lib/i686-pc-linux-gnu/3.0.2/include
> /usr/include
>
Strange.  Using GNU C version 3.0 20010612 (prerelease) I
get /usr/local/include where you get /sw/snapshots/include.