[gentoo-user] Locking dependant installs

Hi,

Suppose the following setup:

I want to emerge application "F", which
depends on library "A","B","C","D" and "E",
which unfortunately are not used by any other 
program and are really heavy to compile in terms
of compile time.

Unfortunately the last step -- the compilation
of "A" -- fails, which I recognize unfortunately
but naturally at the moment all other dependencies
are installed.

Is there a legal and clean way to "lock" those
already installed dependencies and save them
from being wiped out by the tidy and clean up
commands normally used after a general update
of gentoo?

Thank you very much for any help in advance!
Best regards,
mcc