poppler: Explicitly enable/disable boost together with splash

This avoids the following error when splash is not enabled in
PACKAGECONFIG:

  -- Could NOT find Boost (missing: Boost_INCLUDE_DIR) (Required is at
     least version "1.58.0")
  CMake Error at CMakeLists.txt:196 (message):
    -- Boost recommended for Splash.  Use ENABLE_BOOST=OFF to skip.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>