These are the packages that would be merged, in order:

Calculating world dependencies... done!
[ebuild  N    ] dev-php5/pecl-zip-1.0  0 kB 

Total size of downloads: 0 kB

Would you like to merge these packages? [Yes/No] yes

>>> Emerging (1 of 1) dev-php5/pecl-zip-1.0 to /
 * zip-1.0.tgz MD5 ;-) ...                                                                                                                                              [ ok ]
 * zip-1.0.tgz RMD160 ;-) ...                                                                                                                                           [ ok ]
 * zip-1.0.tgz SHA1 ;-) ...                                                                                                                                             [ ok ]
 * zip-1.0.tgz SHA256 ;-) ...                                                                                                                                           [ ok ]
 * zip-1.0.tgz size ;-) ...                                                                                                                                             [ ok ]
 * checking ebuild checksums ;-) ...                                                                                                                                    [ ok ]
 * checking auxfile checksums ;-) ...                                                                                                                                   [ ok ]
 * checking miscfile checksums ;-) ...                                                                                                                                  [ ok ]
 * checking zip-1.0.tgz ;-) ...                                                                                                                                         [ ok ]
>>> Unpacking source...
>>> Unpacking zip-1.0.tgz to /var/tmp/portage/pecl-zip-1.0/work
tar: A lone zero block at 45
>>> Source unpacked.
>>> Compiling source in /var/tmp/portage/pecl-zip-1.0/work/zip-1.0 ...
 * 
 * Using dev-lang/php-5.1.6-r6
 * 
Configuring for:
PHP Api Version:         20041225
Zend Module Api No:      20050922
Zend Extension Api No:   220051025
ac-wrapper: /usr/bin/autoconf-2.13 is missing or not executable.
            Please try emerging the correct version of autoconf.
/usr/portage/eclass/php-ext-source-r1.eclass: line 56: ./configure: No such file or directory

!!! ERROR: dev-php5/pecl-zip-1.0 failed.
Call stack:
  ebuild.sh, line 1546:   Called dyn_compile
  ebuild.sh, line 937:   Called src_compile
  ebuild.sh, line 1255:   Called php-ext-pecl-r1_src_compile
  php-ext-pecl-r1.eclass, line 40:   Called php-ext-source-r1_src_compile
  php-ext-source-r1.eclass, line 56:   Called die

!!! Unable to configure code to compile
!!! If you need support, post the topmost build error, and the call stack if relevant.