cmake: don't include non-IMPORTED ZLIB target in check_library_exists.

check_library_exists/concat does not work well with non-imported targets. ZLIB
might not be an imported target if it is included in a parent cmake project
before including curl.

CMakeLists.txt

@@ -175,6 +175,7 @@ endif()
 
 # initialize CURL_LIBS
 set(CURL_LIBS "")
+set(CURL_DEPS "")
 
 if(ENABLE_ARES)
   set(USE_ARES 1)
@@ -525,7 +526,12 @@ if(ZLIB_FOUND)
   # version of CMake.  This allows our dependents to get our dependencies
   # transitively.
   if(NOT CMAKE_VERSION VERSION_LESS 3.4)
-    list(APPEND CURL_LIBS ZLIB::ZLIB)
+    get_target_property(_imported ZLIB::ZLIB IMPORTED)
+    if(_imported)
+      list(APPEND CURL_LIBS ZLIB::ZLIB)
+    else()
+      list(APPEND CURL_DEPS ZLIB::ZLIB)
+    endif()
   else()
     list(APPEND CURL_LIBS ${ZLIB_LIBRARIES})
     include_directories(${ZLIB_INCLUDE_DIRS})

lib/CMakeLists.txt

@@ -60,7 +60,7 @@ if(ENABLE_CURLDEBUG)
   # being applied to memdebug.c itself.
   set_source_files_properties(memdebug.c PROPERTIES SKIP_UNITY_BUILD_INCLUSION ON)
 endif()
-target_link_libraries(curlu PRIVATE ${CURL_LIBS})
+target_link_libraries(curlu PRIVATE ${CURL_LIBS} ${CURL_DEPS})
 
 transform_makefile_inc("Makefile.soname" "${CMAKE_CURRENT_BINARY_DIR}/Makefile.soname.cmake")
 include(${CMAKE_CURRENT_BINARY_DIR}/Makefile.soname.cmake)

src/CMakeLists.txt

@@ -107,7 +107,7 @@ include_directories(
   )
 
 #Build curl executable
-target_link_libraries(${EXE_NAME} ${LIB_SELECTED_FOR_EXE} ${CURL_LIBS})
+target_link_libraries(${EXE_NAME} ${LIB_SELECTED_FOR_EXE} ${CURL_LIBS} ${CURL_DEPS})
 
 ################################################################################
 

tests/libtest/CMakeLists.txt

@@ -44,7 +44,7 @@ function(setup_test TEST_NAME)          # ARGN are the files in the test
     include_directories(${CARES_INCLUDE_DIR})
   endif()
 
-  target_link_libraries(${TEST_NAME} ${LIB_SELECTED} ${CURL_LIBS})
+  target_link_libraries(${TEST_NAME} ${LIB_SELECTED} ${CURL_LIBS} ${CURL_DEPS})
 
   set_target_properties(${TEST_NAME}
     PROPERTIES COMPILE_DEFINITIONS ${UPPER_TEST_NAME})

tests/server/CMakeLists.txt

@@ -42,7 +42,7 @@ function(SETUP_EXECUTABLE TEST_NAME)    # ARGN are the files in the test
     include_directories(${CARES_INCLUDE_DIR})
   endif()
 
-  target_link_libraries(${TEST_NAME} ${CURL_LIBS})
+  target_link_libraries(${TEST_NAME} ${CURL_LIBS} ${CURL_DEPS})
 
   # Test servers simply are standalone programs that do not use libcurl
   # library.  For convenience and to ease portability of these servers,