Dev flags are now off by default

"Dev" flags are -Werror, -Wextra, -Wall and -Wc++98-compat. People who are
compiling YCM as users and not developers (99.99999% of all YCM compilations)
don't care about those flags and were just burdened by their inclusion by
default.

Fixes #502.

cpp/ycm/CMakeLists.txt

@@ -28,6 +28,7 @@ if ( NOT PYTHONLIBS_VERSION_STRING VERSION_LESS "3.0.0" )
     "python2.\n" )
 endif()
 
+option( DEV_FLAGS "Use compilation flags meant for YCM developers" OFF )
 option( USE_CLANG_COMPLETER "Use Clang semantic completer for C/C++/ObjC" OFF )
 option( USE_SYSTEM_LIBCLANG "Set to ON to use the system libclang library" OFF )
 set( PATH_TO_LLVM_ROOT "" CACHE PATH "Path to the root of a LLVM+Clang binary distribution" )
@@ -295,7 +296,7 @@ set_target_properties( ${PROJECT_NAME} PROPERTIES
 
 # For some reason, Xcode is too dumb to understand the -isystem flag and thus
 # borks on warnings in Boost.
-if ( ( CMAKE_COMPILER_IS_GNUCXX OR COMPILER_IS_CLANG ) AND
+if ( DEV_FLAGS AND ( CMAKE_COMPILER_IS_GNUCXX OR COMPILER_IS_CLANG ) AND
      NOT CMAKE_GENERATOR_IS_XCODE )
   # We want all warnings, and warnings should be treated as errors
   set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror" )
@@ -306,7 +307,7 @@ endif()
 # We want warnings if we accidentally use C++11 features
 # We can't use this warning on FreeBSD because std headers on that OS are dumb.
 # See here: https://github.com/Valloric/YouCompleteMe/issues/260
-if ( COMPILER_IS_CLANG AND NOT CMAKE_GENERATOR_IS_XCODE AND NOT
+if ( DEV_FLAGS AND COMPILER_IS_CLANG AND NOT CMAKE_GENERATOR_IS_XCODE AND NOT
      SYSTEM_IS_FREEBSD )
   set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wc++98-compat" )
 endif()

install.sh

@@ -138,7 +138,7 @@ fi
 if [ -z "$YCM_TESTRUN" ]; then
   install $cmake_args $EXTRA_CMAKE_ARGS
 else
-  testrun $cmake_args $EXTRA_CMAKE_ARGS
+  testrun $cmake_args -DDEV_FLAGS=ON $EXTRA_CMAKE_ARGS
 fi
 
 if $omnisharp_completer; then
@@ -153,7 +153,7 @@ if $omnisharp_completer; then
 
   ycm_dir=`pwd`
   build_dir=$ycm_dir"/python/ycm/completers/cs/OmniSharpServer"
-  
+
   cd $build_dir
   $buildcommand
   cd $ycm_dir