title: why is Xcode / clang translating my "-fno-objc-arc" to "-fno_objc_arc"?
body: <p>i have attempted to follow the advice at SF for turning off objective-C arc per file for one file by placing "-fno-objc-arc" next in the compilation flags for the file in the "Compile Sources" section of my build phases for my project.</p>
  
  <p>this isn't working for me, in that ARC-related errors are still generated.</p>
  
  <p>i see in the log for the file for which i attempted to do this the following warning:</p>
  
  <p>"clang: warning: argument unused during compilation: '-fno_objc_arc'"</p>
  
  <p>i notice that it shows underscores instead of dashes.  when i look at the generated compile line, it also shows underscores instead of dashes.</p>
  
  <pre><code>/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -x objective-c
  -arch armv7 -fmessage-length=0 -std=gnu99 -fobjc-arc -Wno-trigraphs -fpascal-strings -O0
  -Werror-implicit-function-declaration -Wmissing-field-initializers -Wno-missing-prototypes -Wreturn-type
  -Wno-implicit-atomic-properties -Wformat -Wno-missing-braces -Wparentheses -Wswitch -Wno-unused-function
  -Wno-unused-label -Wno-unused-parameter -Wunused-variable -Wunused-value -Wuninitialized -Wno-unknown-pragmas
  -Wno-shadow -Wno-four-char-constants -Wno-sign-compare -Wno-shorten-64-to-32 -Wpointer-sign -Wno-newline-eof
  -Wno-selector -Wno-strict-selector-match -Wundeclared-selector -Wno-deprecated-implementations -DDEBUG=1
  -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk
  -Wprotocol -Wdeprecated-declarations -g -Wno-conversion -Wno-sign-conversion -mthumb "-DIBOutlet=__attribute__((iboutlet))"
  "-DIBOutletCollection(ClassName)=__attribute__((iboutletcollection(ClassName)))" "-DIBAction=void)__attribute__((ibaction)"
  -miphoneos-version-min=5.1 -iquote /x/xcode/DerivedData/myproj-eqyvffptnyujnsgcmhlqunfmfytp/Build/Intermediates/myproj.build/Debug-iphoneos/myproj.build/myproj-generated-files.hmap
  -I/x/xcode/DerivedData/myproj-eqyvffptnyujnsgcmhlqunfmfytp/Build/Intermediates/myproj.build/Debug-iphoneos/myproj.build/myproj-own-target-headers.hmap
  -I/x/xcode/DerivedData/myproj-eqyvffptnyujnsgcmhlqunfmfytp/Build/Intermediates/myproj.build/Debug-iphoneos/myproj.build/myproj-all-target-headers.hmap
  -iquote /x/xcode/DerivedData/myproj-eqyvffptnyujnsgcmhlqunfmfytp/Build/Intermediates/myproj.build/Debug-iphoneos/Voyeur.build/myproj-project-headers.hmap
  -I/x/xcode/DerivedData/myproj-eqyvffptnyujnsgcmhlqunfmfytp/Build/Products/Debug-iphoneos/include
  -I/x/xcode/DerivedData/myproj-eqyvffptnyujnsgcmhlqunfmfytp/Build/Intermediates/myproj.build/Debug-iphoneos/myproj.build/DerivedSources/armv7
  -I/x/xcode/DerivedData/myproj-eqyvffptnyujnsgcmhlqunfmfytp/Build/Intermediates/myproj.build/Debug-iphoneos/myproj.build/DerivedSources
  -F/x/xcode/DerivedData/myproj-eqyvffptnyujnsgcmhlqunfmfytp/Build/Products/Debug-iphoneos -fno_objc_arc
  -include /x/xcode/DerivedData/myproj-eqyvffptnyujnsgcmhlqunfmfytp/Build/Intermediates/PrecompiledHeaders/myproj-Prefix-adnhewmpoabuzebrrqoplkebnugj/myproj-Prefix.pch
  -MMD -MT dependencies -MF /x/xcode/DerivedData/myproj-eqyvffptnyujnsgcmhlqunfmfytp/Build/Intermediates/myproj.build/Debug-iphoneos/myproj.build/Objects-normal/armv7/mySource.d
  --serialize-diagnostics /x/xcode/DerivedData/myproj-eqyvffptnyujnsgcmhlqunfmfytp/Build/Intermediates/myproj.build/Debug-iphoneos/myproj.build/Objects-normal/armv7/mySource.dia
  -c /x/git/myproj/subdir/mySource.m
  -o /x/xcode/DerivedData/myproj-eqyvffptnyujnsgcmhlqunfmfytp/Build/Intermediates/myproj.build/Debug-iphoneos/myproj.build/Objects-normal/armv7/mySource.o
  </code></pre>