fix build of bundled zlib

Ensure that the target for the bundled zlib is only built when the
bundled target is needed.  Also ensure that the build always correctly
finds the necessary headers.  For instance, even if there is a zlib
installed on the system and the build specifies the bundled zlib, ensure
that bundled headers are located before the system headers.

Author: rcsanchez97

CMakeLists.txt

@@ -287,10 +287,6 @@ if (ENABLE_MONGOC)
       ${SOURCE_DIR}/src/zlib-1.2.11/gzread.c
       ${SOURCE_DIR}/src/zlib-1.2.11/gzwrite.c
    )
-   add_library (zlib_obj OBJECT "${ZLIB_SOURCES}")
-   set_property (TARGET zlib_obj PROPERTY POSITION_INDEPENDENT_CODE TRUE)
-   # Disable all warnings for compiling Zlib
-   target_compile_options (zlib_obj PRIVATE -w)
 
    set (MONGOC_ENABLE_ICU 0)
 

src/libmongoc/CMakeLists.txt

@@ -75,7 +75,18 @@ endif ()
 if ( (ENABLE_ZLIB STREQUAL "BUNDLED")
    OR (ENABLE_ZLIB STREQUAL "AUTO" AND NOT ZLIB_FOUND) )
    message (STATUS "Enabling zlib compression (bundled)")
+   add_library (zlib_obj OBJECT "${ZLIB_SOURCES}")
+   set_property (TARGET zlib_obj PROPERTY POSITION_INDEPENDENT_CODE TRUE)
+   # This tells the bundled zlib where to find its generated headers
+   target_include_directories (zlib_obj
+      BEFORE PUBLIC
+      "${SOURCE_DIR}/src/zlib-1.2.11"
+      "${CMAKE_BINARY_DIR}/src/zlib-1.2.11"
+   )
+   # Disable all warnings for compiling Zlib
+   target_compile_options (zlib_obj PRIVATE -w)
    set (SOURCES ${SOURCES} $<TARGET_OBJECTS:zlib_obj>)
+   # This tells mongoc_shared/mongoc_static where to find generated zlib headers
    set (
       ZLIB_INCLUDE_DIRS
       "${SOURCE_DIR}/src/zlib-1.2.11"