[Feature] Add compiler flags per files on Xcode Project with addon_config.mk.

I can't show a better way how to specify flags to file.

But I found where is compiler flags in project.xcodeproj.

const char PBXBuildFile[] =
STRINGIFY(
          <key>BUILDUUID</key>
          <dict>
              <key>fileRef</key>
              <string>FILEUUID</string>
              <key>isa</key>
              <string>PBXBuildFile</string>
+              <key>settings</key>
+              <dict>
+                  <key>COMPILER_FLAGS</key>
+                  <string>__COMPILER_FLAGS__</string>
+              </dict>
          </dict>
);

idea of how to specify flags:

variable: ADDON_COMPILERFLAGS_PER_FILE
values: SOURCE_FILE COMPILER_FLAG

e.g.

ADDON_COMPILERFLAGS_PER_FILE = src/foo.m -fno-objc-arc
ADDON_COMPILERFLAGS_PER_FILE += src/foo.m -DHOGE
ADDON_COMPILERFLAGS_PER_FILE += src/bar.m -fobjc-arc

those give generic feature.

but, (maybe) mainly we want to use for adding -fno-objc-arc or -fobjc-arc.
so, other way is like

ADDON_NO_OBJC_ARC = SOURCE_FILE
ADDON_OBJC_ARC = SOURCE_FILE

Probably this is easy to implement than generic way.

Start with addon_config.mk and the PBXBuildFile section of the generated project.xcodeproj format described in the issue. Resolve whether the feature should support generic per-file compiler flags, ARC-specific variables, or both, then trace the generator entry point that reads addon_config.mk. Done means the selected configuration produces the expected per-file COMPILER_FLAGS entries.