Fixes a build failure with CMake 2.8.12.

Discussed in http://article.gmane.org/gmane.comp.cad.brlcad.devel/3782

Index: src/other/CMakeLists.txt
===================================================================
--- src/other/CMakeLists.txt	(revision 58323)
+++ src/other/CMakeLists.txt	(working copy)
@@ -390,6 +390,7 @@
   # PNG_INCLUDE_DIR, so a generic macro won't pick up the setting -
   # handle it here.
 
+  set(PNG_LIBRARY "${PNG_LIBRARY}" CACHE STRING "PNG library" FORCE)
   set(PNG_INCLUDE_DIR ${PNG_PNG_INCLUDE_DIR} CACHE STRING "PNG include directory" FORCE)
 endif(BRLCAD_PNG_BUILD)
 DISTCLEAN(${CMAKE_CURRENT_BINARY_DIR}/libpng/CTestTestfile.cmake)