Add a way to suppress the PNG use of 'd' suffix

Grr.  If build type gets set to Debug, instead of being left empty,
Linux is also throwing in the 'd' suffix on debug build libs from png.
Add a way to specify at configure time not to do that - it really messes
with our build logic.  Should probably try to upstream this or something
like it.

src/other/ext/png.cmake

@@ -89,6 +89,7 @@ if (BRLCAD_PNG_BUILD)
     -DCMAKE_INSTALL_RPATH_USE_LINK_PATH=${CMAKE_INSTALL_RPATH_USE_LINK_PATH}
     -DCMAKE_SKIP_BUILD_RPATH=${CMAKE_SKIP_BUILD_RPATH}
     -DPNG_LIB_NAME=${PNG_LIB_NAME}
+    -DPNG_NO_DEBUG_POSTFIX=ON
     -DPNG_PREFIX=brl_
     -DPNG_STATIC=${BUILD_STATIC_LIBS}
     -DPNG_TESTS=OFF

src/other/ext/png/CMakeLists.txt

@@ -224,7 +224,9 @@ endif(PNG_HARDWARE_OPTIMIZATIONS)
 set(PNG_LIB_NAME png${PNGLIB_MAJOR}${PNGLIB_MINOR})
 
 # Distinguish between debug and release builds.
-set(CMAKE_DEBUG_POSTFIX "d")
+if(NOT PNG_NO_DEBUG_POSTFIX)
+	set(CMAKE_DEBUG_POSTFIX "d")
+endif(NOT PNG_NO_DEBUG_POSTFIX)
 
 include(CheckCSourceCompiles)
 option(ld-version-script "Enable linker version script" ON)