N900 root file system doesn't have enough space for full-blown C++ and all stuff. Well, if you really want to develop on N900 itself, then you have a choice of two:

1) install chroot-debian (or so - do a search). It allows you to execute "build-essentials" under chroot in 2GB partition.

2) Use this thread - http://talk.maemo.org/showthread.php?t=59374 which installs your Maemo5 into 2GB partition (I use this more one year) and you would have no space limits.

After that - install "build-essential" under chroot or just directly in new root FS.

BTW, if you have plan to build apps with Qt4, then look into this - https://garage.maemo.org/projects/libqt4-dev-arm/
It has an ARM binaries to run Qt4 on your N900 (look inside for explanations). I used it in past to build google-latitude.