build: PNG auto-build improvements (AcademySoftwareFoundation#4835)

The main thing is no longer using a custom namespace prefix for
auto-built libpng. We're already building it statically by default and
hiding the symbols, so the custom prefix only serves to make occasional
build problems by having different symbols than typically found in the
library. We still keep it as an option that can be enabled.

Also, be a little more careful in clearing unneeded variable that may
have been set by an earier attempt to find the package but that failed
to turn up a version we were happy with.

---------

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/cmake/build_PNG.cmake

@@ -7,26 +7,34 @@
 ######################################################################
 
 set_cache (PNG_BUILD_VERSION 1.6.47 "PNG version for local builds")
-set (PNG_GIT_REPOSITORY "https://github.com/pnggroup/libpng")
-set (PNG_GIT_TAG "v${PNG_BUILD_VERSION}")
-
+super_set (PNG_BUILD_GIT_REPOSITORY "https://github.com/pnggroup/libpng")
+super_set (PNG_BUILD_GIT_TAG "v${PNG_BUILD_VERSION}")
+super_set (PNG_BUILD_EXTRA_CMAKE_ARGS "")
 set_cache (PNG_BUILD_SHARED_LIBS OFF
            DOC "Should execute a local PNG build, if necessary, build shared libraries" ADVANCED)
+set_option (PNG_BUILD_USE_CUSTOM_PREFIX "Use custom namespace prefix for libpng" OFF)
 
+if (PNG_BUILD_USE_CUSTOM_PREFIX)
+    list (APPEND PNG_BUILD_EXTRA_CMAKE_ARGS -D PNG_PREFIX=oiio)
+endif ()
 string (MAKE_C_IDENTIFIER ${PNG_BUILD_VERSION} PNG_VERSION_IDENT)
 
 unset (PNG_FOUND)
+unset (PNG_LIBRARY)
+unset (PNG_LIBRARY_RELEASE)
+unset (PNG_LIBRARY_DEBUG)
 unset (PNG_LIBRARIES)
 unset (PNG_INCLUDE_DIRS)
 unset (PNG_INCLUDE_DIR)
+unset (PNG_PNG_INCLUDE_DIR)
 unset (PNG_VERSION_STRING)
 unset (PNG_DEFINITIONS)
 unset (PNG_VERSION)
 
 build_dependency_with_cmake (PNG
     VERSION         ${PNG_BUILD_VERSION}
-    GIT_REPOSITORY  ${PNG_GIT_REPOSITORY}
-    GIT_TAG         ${PNG_GIT_TAG}
+    GIT_REPOSITORY  ${PNG_BUILD_GIT_REPOSITORY}
+    GIT_TAG         ${PNG_BUILD_GIT_TAG}
     CMAKE_ARGS
         -D PNG_SHARED=${PNG_BUILD_SHARED_LIBS}
         -D PNG_STATIC=ON
@@ -36,17 +44,25 @@ build_dependency_with_cmake (PNG
         -D PNG_FRAMEWORK=OFF
         -D CMAKE_POSITION_INDEPENDENT_CODE=ON
         -D CMAKE_INSTALL_LIBDIR=lib
-        -D PNG_PREFIX=oiio
+        ${PNG_BUILD_EXTRA_CMAKE_ARGS}
     )
 
 
 set (PNG_REFIND FALSE)
 set (PNG_REFIND_VERSION ${PNG_BUILD_VERSION})
-
-
 set (PNG_REFIND_ARGS EXACT REQUIRED)
-set (PNG_DIR ${PNG_LOCAL_INSTALL_DIR}/lib/cmake/PNG)
 set (PNG_FIND_VERSION_EXACT ON)
+unset (PNG_FOUND)
+unset (PNG_LIBRARY)
+unset (PNG_LIBRARY_RELEASE)
+unset (PNG_LIBRARY_DEBUG)
+unset (PNG_LIBRARIES)
+unset (PNG_INCLUDE_DIRS)
+unset (PNG_INCLUDE_DIR)
+unset (PNG_PNG_INCLUDE_DIR)
+unset (PNG_VERSION_STRING)
+unset (PNG_DEFINITIONS)
+unset (PNG_VERSION)
 
 if (PNG_BUILD_VERSION VERSION_GREATER 1.6.43)
     list (APPEND PNG_REFIND_ARGS CONFIG)