pitter patter on the keyboard

After upgrading libpng you may encounter this error when compiling packages that depend on it or related libraries:

/usr/lib/gcc/i686-pc-linux-gnu/4.5.3/../../../../i686-pc-linux-gnu/bin/ld: cannot find -lpng12
collect2: ld returned 1 exit status
make[2]: *** [librsvg-2.la] Error 1
make[2]: Leaving directory `/var/tmp/portage/gnome-base/librsvg-2.34.1-r1/work/librsvg-2.34.1'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/var/tmp/portage/gnome-base/librsvg-2.34.1-r1/work/librsvg-2.34.1'
make: *** [all] Error 2
 * ERROR: gnome-base/librsvg-2.34.1-r1 failed (compile phase):
 *   emake failed
 * 
 * If you need support, post the output of 'emerge --info =gnome-base/librsvg-2.34.1-r1',
 * the complete build log and the output of 'emerge -pqv =gnome-base/librsvg-2.34.1-r1'.
 * The complete build log is located at '/var/tmp/portage/gnome-base/librsvg-2.34.1-r1/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/gnome-base/librsvg-2.34.1-r1/temp/environment'.
 * S: '/var/tmp/portage/gnome-base/librsvg-2.34.1-r1/work/librsvg-2.34.1'

Resolve by recompiling reverse dependencies:

# revdep-rebuild --library libpng12.so.0

If you recently updated libpng you may encounter the following error when compiling packages depending on cairo, such as librsvg:

Package 'libpng12', required by 'cairo', not found

Update cairo before proceeding:

# emerge --update --deep cairo