This page i am trying to cover what ever i have found during my setup and issue.

EXTRA_ECONF="--with-apxs=/home/apache/bin/apxs" emerge -O mod_security

Right now, users can replace the defaults:

 prefix, host, mandir, infodir, datadir, sysconfdir, localstatedir and
 datadir, infodir, localstatedir, mandir, sysconfir

add:

 anything that doesn't collide with additional arguments in the ebuild.

emerge -pv apg

emerge --sync

Masked is not always broken. But, make sure you are sure to use it. First define your arch. i am using “amd64”
you can usually find following options

~alpha ~amd64 ~arm ~hppa ~ia64 ~m68k ~mips ~ppc ~ppc64 ~s390 ~sh ~sparc ~sparc-fbsd ~x86 ~x86-fbsd

NOTE : “~” is not a seperator it is important element here.
Now i want to build “bash-4.0_p28” but it is masked.
i need to use it following way to build.

ACCEPT_KEYWORDS="~amd64" emerge bash

you see i have defined whole word “~amd64”.
this will build the bash to the latest.