Fix inline if/else statement in CMAKE_CACHE_ARGS

An if/else statement was given inline as an argument to
CMAKE_CACHE_ARGS for some CMake external projects. This
resulted in the following init cache entries on some systems:

  set(CMAKE_POSITION_INDEPENDENT_CODE "ON;if;(;tensorflow_
  	ENABLE_POSITION_INDEPENDENT_CODE;);else;(;)"
  	CACHE BOOL "Initial cache" FORCE)
  set(CMAKE_POSITION_INDEPENDENT_CODE "OFF;endif;(;)" CACHE
  	BOOL "Initial cache" FORCE)

This commit changes the inline if/else argument to
-DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=
${tensorflow_ENABLE_POSITION_INDEPENDENT_CODE} which
is functionality equivalent.