CMake: Generate pkg-config files for all Windows builds

The Meson build system makes use of pkg-config files or CMake to find most of
the dependencies via its "built-in" dependency search system.  As libpng is a
very widely used package, this makes the CMake build system generate pkg-config
files for libpng even for non-Cygwin/non-MinGW Windows builds, so that Meson
could locate libpng via pkg-config as well even on Visual Studio builds, for
instance.

We also do the following on native, non-MinGW Windows builds:

*  Replace -lpngXX with libpngXX.lib, for the libpng library that we link to
*  Use the correct ZLib library .lib name for `libs.private`.
*  Replace `Requires: zlib` with `Requires:`, since we already say in
   `libs.private` that we are linking to ZLib

CMakeLists.txt

@@ -871,31 +871,35 @@ endif()
 
 # Create pkgconfig files.
 # We use the same files like ./configure, so we have to set its vars.
-# Only do this on Windows for Cygwin - the files don't make much sense
-# outside of a UNIX look-alike.
+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(prefix      ${CMAKE_INSTALL_PREFIX})
-  set(exec_prefix ${CMAKE_INSTALL_PREFIX})
-  set(libdir      ${CMAKE_INSTALL_FULL_LIBDIR})
-  set(includedir  ${CMAKE_INSTALL_FULL_INCLUDEDIR})
-  set(LIBS        "-lz -lm")
-  configure_file(${CMAKE_CURRENT_SOURCE_DIR}/libpng.pc.in
-                 ${CMAKE_CURRENT_BINARY_DIR}/${PNGLIB_NAME}.pc
-                 @ONLY)
-  create_symlink(libpng.pc FILE ${PNGLIB_NAME}.pc)
-  configure_file(${CMAKE_CURRENT_SOURCE_DIR}/libpng-config.in
-                 ${CMAKE_CURRENT_BINARY_DIR}/${PNGLIB_NAME}-config
-                 @ONLY)
-  create_symlink(libpng-config FILE ${PNGLIB_NAME}-config)
+  set(LIBS      "-lz -lm")
+else()
+  STRING (TOLOWER "${CMAKE_BUILD_TYPE}" config_lower)
+  if (config_lower STREQUAL "debug")
+    set(LIBS    "${ZLIB_LIBRARY_DEBUG} ${M_LIBRARY}")
+  else()
+    set(LIBS    "${ZLIB_LIBRARY_RELEASE} ${M_LIBRARY}")
+  endif()
 endif()
+configure_file(${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)
-  set_target_properties(png PROPERTIES
-    VERSION 16.${PNGLIB_RELEASE}.git
-#   VERSION 16.${PNGLIB_RELEASE}.0
-    SOVERSION 16
-    CLEAN_DIRECT_OUTPUT 1)
+if(NOT WIN32 OR CYGWIN OR MINGW)
+  configure_file(${CMAKE_CURRENT_SOURCE_DIR}/libpng-config.in
+    pc_file${PNGLIB_NAME}-config @ONLY)
+  create_symlink(libpng-config FILE ${PNGLIB_NAME}-config)
+else()
+  foreach (pc_file ${CMAKE_CURRENT_BINARY_DIR}/${PNGLIB_NAME}.pc ${CMAKE_CURRENT_BINARY_DIR}/libpng.pc)
+    FILE(READ ${pc_file} png_pc_orig)
+    STRING(REPLACE "-lpng${PNGLIB_MAJOR}${PNGLIB_MINOR}" "libpng${PNGLIB_MAJOR}${PNGLIB_MINOR}.lib" png_pc_out ${png_pc_orig})
+    STRING(REPLACE "Requires: zlib" "Requires:" png_pc_out ${png_pc_out})
+    FILE(WRITE ${pc_file} ${png_pc_out})
+  endforeach()
 endif()
 
 # Install.
@@ -958,13 +962,13 @@ if(NOT SKIP_INSTALL_FILES AND NOT SKIP_INSTALL_ALL)
   install(FILES png.5
           DESTINATION ${CMAKE_INSTALL_MANDIR}/man5)
   # Install the pkg-config files.
+  install(FILES ${CMAKE_CURRENT_BINARY_DIR}/libpng.pc
+          DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)
+  install(FILES ${CMAKE_CURRENT_BINARY_DIR}/${PNGLIB_NAME}.pc
+          DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)
   if(NOT CMAKE_HOST_WIN32 OR CYGWIN OR MINGW)
-    install(FILES ${CMAKE_CURRENT_BINARY_DIR}/libpng.pc
-            DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)
     install(PROGRAMS ${CMAKE_CURRENT_BINARY_DIR}/libpng-config
             DESTINATION ${CMAKE_INSTALL_BINDIR})
-    install(FILES ${CMAKE_CURRENT_BINARY_DIR}/${PNGLIB_NAME}.pc
-            DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)
     install(PROGRAMS ${CMAKE_CURRENT_BINARY_DIR}/${PNGLIB_NAME}-config
             DESTINATION ${CMAKE_INSTALL_BINDIR})
   endif()