Add options and instructions for seeing line numbers in sanitizer backtraces

Author: sodabrew

README.md

@@ -64,6 +64,13 @@ all sanitizers or fail if none are available. If a command-separated list of
 specific sanitizers is given, configure will fail unless they all are available.
 Note that the some sanitizers may incur a performance penalty, and the Address
 Sanitizer may require a runtime library.
+To see line numbers in backtraces, declare these environment variables
+(adjust the llvm-symbolizer path as needed for your system):
+
+``` sh
+  export ASAN_SYMBOLIZER_PATH=/usr/bin/llvm-symbolizer-3.4
+  export ASAN_OPTIONS=symbolize=1
+```
 
 ### Linux and other Unixes
 

ext/mysql2/extconf.rb

@@ -150,6 +150,8 @@ def asplode(lib)
     end
     $CFLAGS << " -fsanitize=#{s}"
   end
+  # Options for line numbers in backtraces
+  $CFLAGS << ' -g -fno-omit-frame-pointer'
 end
 
 if RUBY_PLATFORM =~ /mswin|mingw/