This problem was reported before in bug #56 but the root cause wasn't solved as explained in the last comment.

The problem is that the PKG_PROG_PKG_CONFIG is defined as part of the calls to check for Truetype or PNG support (through PKG_CHECK_EXISTS) and that code is called conditionally depending on the flags that were passed to configure, therefore resulting on side effects like getting png support disabled when --disable-xft is used as shown by (from any release, including HEAD on git) :

  $ ./configure --disable-xft | grep png | grep checking
  checking for libpng12... checking for libpng... no
  $ ./configure | grep png | grep checking   
  checking for libpng12... no
  checking for libpng... yes

the solution as spelled out in acinclude.m4 is to add an explicit call for PKG_PROG_PKG_CONFIG in configure.ac