heads up: netbsd and building with lang/gcc* packages

[maya%netbsd.org@localhost]

Hi pkgsrc-users who are netbsd users who have chosen to build things
with lang/gcc* packages.

I've changed the lang/gcc{48,49,5,6,7} packages to create libstdc++.so.7
rather than libstdc++.so.6. this means a binary built with that compiler
may fail with 'missing library libstdc++.so.6'.

I've done this because the libstdc++ that comes with netbsd is called
libstdc++.so.7, and before, both libraries would be used. libraries
aren't built to support this usage, but I've not heard of suspicious
failures.

If you are seeing this, you will need to rebuild the package.

being a compiler, you can technically build the entire tree with
lang/gcc* compilers. or none of it, if your compiler is new enough.

I've made it seem like firefox{,45,52} have had an update, so updates
will rebuild those. they're the most common case of using a newer
compiler on netbsd-7.

all of the above is netbsd specific. there is no change for non-netbsd.