You appear to have set $CFLAGS, perhaps you also need to tell GMP the
intended ABI, see "ABI and ISA" in the manual.

!!! Please attach the following file when seeking support:
!!! /var/tmp/portage/dev-libs/gmp-4.2.2/work/gmp-4.2.2/config.log
 *
 * ERROR: dev-libs/gmp-4.2.2 failed.
 * Call stack:
 *               ebuild.sh, line   49:  Called src_compile
 *             environment, line 2618:  Called econf 'src_compile' 'src_compile' '--localstatedir=/var/state/gmp' '--disable-mpfr'
 *               ebuild.sh, line  513:  Called die
 * The specific snippet of code:
 *                      die "econf failed"
 *  The die message:
 *   econf failed
 *
 * If you need support, post the topmost build error, and the call stack if relevant.
 * A complete build log is located at '/var/tmp/portage/dev-libs/gmp-4.2.2/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-libs/gmp-4.2.2/temp/environment'.
 *

 * Messages for package dev-libs/gmp-4.2.2:

 *
 * ERROR: dev-libs/gmp-4.2.2 failed.
 * Call stack:
 *               ebuild.sh, line   49:  Called src_compile
 *             environment, line 2618:  Called econf 'src_compile' 'src_compile' '--localstatedir=/var/state/gmp' '--disable-mpfr'
 *               ebuild.sh, line  513:  Called die
 * The specific snippet of code:
 *                      die "econf failed"
 *  The die message:
 *   econf failed
 *
 * If you need support, post the topmost build error, and the call stack if relevant.
 * A complete build log is located at '/var/tmp/portage/dev-libs/gmp-4.2.2/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-libs/gmp-4.2.2/temp/environment'.
 *
 * Regenerating GNU info directory index...
 * Processed 78 info files.