CMakeLists.txt: Improve pkg-config generating on Windows

Use the target properties of ZLIB::ZLIB to get the library names for the
pkg-config files that we generate, and unify things with other builds.
We need to use the IMPORT_LOCATION[_$<CONFIG>] for ZLib since it does not
define IMPORT_IMPLIB[_$<CONFIG>] even on Windows DLL builds, and assume
that the $<CONFIG> we use above is RELEASE for both MinSizeRel and
RelWithDebInfo builds.

Sadly, this pkg-config generating will not work currently with the Visual
Studio project generators, since CMAKE_BUILD_TYPE is usually not defined in
such builds.

CMakeLists.txt

@@ -939,23 +939,26 @@ endif()
 
 # Create pkgconfig files.
 # We use the same files like ./configure, so we have to set its vars.
-set(prefix      ${CMAKE_INSTALL_PREFIX})
-set(exec_prefix ${CMAKE_INSTALL_PREFIX})
-set(libdir      ${CMAKE_INSTALL_FULL_LIBDIR})
-set(includedir  ${CMAKE_INSTALL_FULL_INCLUDEDIR})
-if(NOT WIN32 OR CYGWIN OR MINGW)
-  set(LIBS      "-lz -lm")
-else()
-  string(TOLOWER "${CMAKE_BUILD_TYPE}" config_lower)
-  if (config_lower STREQUAL "debug")
-    set(LIBS    "${ZLIB_LIBRARY_DEBUG} ${M_LIBRARY}")
-  else()
-    set(LIBS    "${ZLIB_LIBRARY_RELEASE} ${M_LIBRARY}")
+if(NOT "${CMAKE_BUILD_TYPE}" STREQUAL "")
+  set(prefix      ${CMAKE_INSTALL_PREFIX})
+  set(exec_prefix ${CMAKE_INSTALL_PREFIX})
+  set(libdir      ${CMAKE_INSTALL_FULL_LIBDIR})
+  set(includedir  ${CMAKE_INSTALL_FULL_INCLUDEDIR})
+  get_target_property(ZLIB_LIB ZLIB::ZLIB IMPORTED_LOCATION)
+  if("${ZLIB_LIB}" STREQUAL "ZLIB_LIB-NOTFOUND")
+    # Sadly, no IMPORTED_LOCATION_RELWITHDEBINFO property in ZLIB::ZLIB?
+    if("${CMAKE_BUILD_TYPE}" STREQUAL "RelWithDebInfo" OR
+       "${CMAKE_BUILD_TYPE}" STREQUAL "MinSizeRel")
+      get_target_property(ZLIB_LIB ZLIB::ZLIB IMPORTED_LOCATION_RELEASE)
+    else()
+      get_target_property(ZLIB_LIB ZLIB::ZLIB IMPORTED_LOCATION_${ZLIB_CFG})
+    endif()
   endif()
+  set(LIBS    "${ZLIB_LIB} ${M_LIBRARY}")
+  configure_file(${CMAKE_CURRENT_SOURCE_DIR}/libpng.pc.in
+    ${CMAKE_CURRENT_BINARY_DIR}/${PNGLIB_NAME}.pc @ONLY)
+  create_symlink(libpng.pc FILE ${PNGLIB_NAME}.pc)
 endif()
-configure_file(${CMAKE_CURRENT_SOURCE_DIR}/libpng.pc.in
-  ${CMAKE_CURRENT_BINARY_DIR}/${PNGLIB_NAME}.pc @ONLY)
-create_symlink(libpng.pc FILE ${PNGLIB_NAME}.pc)
 
 # Set up links.
 if(PNG_SHARED)