- don't hardcode optimization flags and drop -Werror

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -38,7 +38,7 @@ if(MSVC)
     string(REGEX REPLACE "/RTC(su|[1su])" "" ${flag_var} "${${flag_var}}")
   endforeach(flag_var)
 else()
-  add_compile_options(-Wall -Werror -O3)
+  add_compile_options(-Wall)
 endif()
 
 if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)