update podspec to support all 4 standalone components, including `enc`, `dec`, `demux` and `mux`, all of these subspecs can be compiled without extra dependency

libwebp.podspec

@@ -19,9 +19,42 @@ Pod::Spec.new do |s|
     'USER_HEADER_SEARCH_PATHS' => '$(inherited) ${PODS_ROOT}/libwebp/ ${PODS_TARGET_SRCROOT}/'
   }
   s.preserve_path = 'src'
+  s.default_subspecs = 'dec', 'enc', 'demux', 'mux'
 
-  s.source_files = 'src/webp/*.{h,c}', 'src/utils/*.{h,c}', 'src/dsp/*.{h,c}', 'src/enc/*.{h,c}', 'src/dec/*.{h,c}', 'src/demux/*.{h,c}', 'src/mux/*.{h,c}'
-  s.public_header_files = 'src/webp/*.h'
+  # common code, used by actual subspecs
+  s.subspec 'core' do |ss|
+    ss.source_files = 'src/utils/*.{h,c}', 'src/dsp/*.{h,c}', 'src/webp/types.h', 'src/webp/format_constants.h'
+    ss.public_header_files = 'src/webp/types.h', 'src/webp/format_constants.h'
+  end
+
+  # webp decoding
+  s.subspec 'dec' do |ss|
+    ss.dependency 'libwebp/core'
+    ss.source_files = 'src/dec/*.{h,c}', 'src/webp/decode.h'
+    ss.public_header_files = 'src/webp/decode.h'
+  end
+
+  # webp encoding
+  s.subspec 'enc' do |ss|
+    ss.dependency 'libwebp/core'
+    ss.source_files = 'src/enc/*.{h,c}', 'src/webp/encode.h'
+    ss.public_header_files = 'src/webp/encode.h'
+  end
+
+  # animated webp decoding
+  s.subspec 'demux' do |ss|
+    ss.dependency 'libwebp/dec'
+    ss.source_files = 'src/demux/*.{h,c}', 'src/webp/demux.h', 'src/webp/mux_types.h'
+    ss.public_header_files = 'src/webp/demux.h', 'src/webp/mux_types.h'
+  end
+
+  # animated webp encoding
+  s.subspec 'mux' do |ss|
+    ss.dependency 'libwebp/enc'
+    ss.dependency 'libwebp/demux'
+    ss.source_files = 'src/mux/*.{h,c}', 'src/webp/mux.h'
+    ss.public_header_files = 'src/webp/mux.h'
+  end
 
   # fix #include <inttypes.h> cause 'Include of non-modular header inside framework module error'
   s.prepare_command = <<-CMD