Build: Set CMP0065 NEW (respect ENABLE_EXPORTS)

Referring to https://cmake.org/cmake/help/latest/policy/CMP0065.html,
CMake 3.3 and earlier automatically added compiler/linker flags such as
-rdynamic/-export-dynamic, which caused symbols to be exported from
executables.  The primary purpose of this is to allow plugins loaded via
dlopen() to access symbols from the calling program.  libjpeg-turbo
does not need this functionality, and enabling it needlessly increases
the size of the libjpeg-turbo executables.

Setting CMP0065 to NEW when using CMake 3.4 and later prevents CMake
from automatically adding the aforementioned compiler/linker flags
unless the ENABLE_EXPORTS property is set for a target (or the
CMAKE_ENABLE_EXPORTS variable is set, which causes ENABLE_EXPORTS to be
set for all targets.)

Closes #554

CMakeLists.txt

@@ -1,4 +1,9 @@
 cmake_minimum_required(VERSION 2.8.12)
+# When using CMake 3.4 and later, don't export symbols from executables unless
+# the CMAKE_ENABLE_EXPORTS variable is set.
+if(POLICY CMP0065)
+  cmake_policy(SET CMP0065 NEW)
+endif()
 
 if(CMAKE_EXECUTABLE_SUFFIX)
   set(CMAKE_EXECUTABLE_SUFFIX_TMP ${CMAKE_EXECUTABLE_SUFFIX})