Make in-tree zlib more robust.

.gitmodules

@@ -1,6 +1,7 @@
 [submodule "extlib/zlib"]
 	path = extlib/zlib
 	url = https://github.com/madler/zlib
+	ignore = dirty
 [submodule "extlib/libpng"]
 	path = extlib/libpng
 	url = https://github.com/glennrp/libpng

CMakeLists.txt

@@ -63,33 +63,36 @@ if(NOT (PERL_FOUND AND PERLMODULES_FOUND))
 endif()
 
 if(WIN32)
-    find_package(PNG)
+    find_package(ZLIB)
 
-    if(NOT PNG_FOUND)
-        message(STATUS "Using in-tree libpng")
+    if(NOT ZLIB_FOUND)
+        message(STATUS "Using in-tree zlib")
 
         add_subdirectory(extlib/zlib)
 
-        message(STATUS "Using in-tree libpng")
-
         set(ZLIB_LIBRARY
             zlibstatic)
         set(ZLIB_INCLUDE_DIR
             "${CMAKE_SOURCE_DIR}/extlib/zlib"
             "${CMAKE_BINARY_DIR}/extlib/zlib")
+        find_package(ZLIB REQUIRED)
+    endif()
+
+    find_package(PNG)
+
+    if(NOT PNG_FOUND)
+        message(STATUS "Using in-tree libpng")
+
         set(SKIP_INSTALL_ALL
             ON)
         add_subdirectory(extlib/libpng)
 
-        set(PNG_FOUND TRUE)
-        set(PNG_LIBRARIES
-            png16_static
-            zlibstatic)
-        set(PNG_INCLUDE_DIRS
+        set(PNG_LIBRARY
+            png16_static)
+        set(PNG_PNG_INCLUDE_DIR
             "${CMAKE_SOURCE_DIR}/extlib/libpng"
-            "${CMAKE_BINARY_DIR}/extlib/libpng"
-            "${CMAKE_SOURCE_DIR}/extlib/zlib"
-            "${CMAKE_BINARY_DIR}/extlib/zlib")
+            "${CMAKE_BINARY_DIR}/extlib/libpng")
+        find_package(PNG REQUIRED)
     endif()
 
     if(NOT MINGW)

src/CMakeLists.txt

@@ -149,7 +149,7 @@ elseif(APPLE)
         unix/solvespace-48x48.png)
 
     set(platform_BUNDLED_LIBS
-        ${PNG_LIBRARY})
+        ${PNG_LIBRARIES})
 
     set(platform_LIBRARIES
         ${APPKIT_LIBRARY})