PERF: Allow overwriting ABI/Optimization & Warning flags
PERF: Allow overwriting ABI/Optimization & Warning flags

Provide a mechanism for changing the default warning
and ABI/OPTIMIZATION flags.

The default ABI/Optimization flag settings are
set to:
    -mtune=native # Tune the code for the computer used compile
                  # ITK, but allow running on generic cpu
                  # archetectures
    -march=corei7 # Use ABI settings to support corei7
                  # (circa 2008 ABI feature sets)

The defaults can easily be overriden from the cmake comand line with:
  -DITK_C_OPTIMIZATION_FLAGS:STRING="-march=native"
  -DITK_CXX_OPTIMIZATION_FLAGS:STRING="-march=native"
  -DITK_C_WARNING_FLAGS:STRING="-Wall -Werror"
  -DITK_CXX_WARNING_FLAGS:STRING="-Wall -Werror"

richardbeare/ITK,LucasGandel/ITK,vfonov/ITK,hjmjohnson/ITK,thewtex/ITK,hjmjohnson/ITK,thewtex/ITK,thewtex/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,malaterre/ITK,vfonov/ITK,richardbeare/ITK,thewtex/ITK,vfonov/ITK,LucasGandel/ITK,malaterre/ITK,vfonov/ITK,thewtex/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,malaterre/ITK,BRAINSia/ITK,vfonov/ITK,Kitware/ITK,richardbeare/ITK,hjmjohnson/ITK,thewtex/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,BRAINSia/ITK,LucasGandel/ITK,richardbeare/ITK,richardbeare/ITK,LucasGandel/ITK,BRAINSia/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,BRAINSia/ITK,hjmjohnson/ITK,hjmjohnson/ITK,malaterre/ITK,Kitware/ITK,hjmjohnson/ITK,malaterre/ITK,hjmjohnson/ITK,malaterre/ITK,malaterre/ITK,thewtex/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,BRAINSia/ITK,Kitware/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,BRAINSia/ITK,vfonov/ITK,LucasGandel/ITK,BRAINSia/ITK,Kitware/ITK,vfonov/ITK,richardbeare/ITK