Make LTO opt-in by default instead of opt-out

LTO has caused problems to a few users (see issues linked at the
bottom).
The problem is the runtime is built as a static library by default
unless otherwise specified via CMake flags, and also LTO is enabled by
default. Those two things combined means the user must turn on LTO when
they build their application if they're using GCC.

Since we don't have evidence that LTO is making a substantial difference
in the runtime, it seems prudent to leave the option but turn if off by
default.

Issues where this has been reported:
awslabs#151
awslabs#128

Author: marcomagdy

CMakeLists.txt

@@ -4,7 +4,7 @@ project(aws-lambda-runtime
     VERSION 0.0.0
     LANGUAGES CXX)
 
-option(ENABLE_LTO "Enables link-time optimization, requires compiler support." ON)
+option(ENABLE_LTO "Enables link-time optimization, requires compiler support." OFF)
 option(ENABLE_TESTS "Enables building the test project, requires AWS C++ SDK." OFF)
 
 add_library(${PROJECT_NAME}