If you try to install a PECL package without using Portage (thus using the PHP tool
pecl), you might encounter an error like this snippet:
bender ~ # pecl install apc downloading APC-3.0.8.tgz ... [...] autoconf: Undefined macros: configure.in:63:AC_PROG_LIBTOOL ERROR: `phpize' failed
The main problem lies with the use of automake v1.9.x. Since Gentoo comes with a bunch of different versions of the autotools, you can choose to use automake v1.8, which will result in a complete built.
bender ~ # WANT_AUTOMAKE="1.8" pecl install apc