Fix the androideabi toolchain not matching host

The included method of determining the host platform in the androideabi
would not match to windows if RUBY_PLATFORM returned mingw, wince, or
emx. As an added pre-caution, using a case statement with regexp
matching will better determine the host platform since it won't be case
sensitive and for windows is set to match more specifially than just
"win".

tasks/toolchains/androideabi.rake

@@ -41,14 +41,15 @@ MRuby::Toolchain.new(:androideabi) do |conf|
   end
 
   if ANDROID_STANDALONE_TOOLCHAIN == nil then
-    if RUBY_PLATFORM.include?('darwin') then
+    case RUBY_PLATFORM
+    when /cygwin|mswin|mingw|bccwin|wince|emx/i
+      HOST_PLATFORM = 'windows'
+    when /darwin/i
       HOST_PLATFORM = 'darwin-x86'
-    elsif RUBY_PLATFORM.include?('linux') then
+    when /linux/i
       HOST_PLATFORM = 'linux-x86'
-    elsif RUBY_PLATFORM.include?('win') then
-      HOST_PLATFORM = 'windows'
     else
-      # Unknown host platform.
+      # Unknown host platform
     end
 
     ANDROID_TARGET_PLATFORM = ENV['ANDROID_TARGET_PLATFORM'] || DEFAULT_ANDROID_TARGET_PLATFORM