When I run "cmake -DENABLE_DCTDECODER=none -DENABLE_BOOST=OFF" on my
system, I get the following output:

    -- Found Iconv: /usr/lib/x86_64-linux-gnu/libc.so
    CMake Warning at CMakeLists.txt:252 (find_package):
      By not providing "FindOpenJPEG.cmake" in CMAKE_MODULE_PATH this project has
      asked CMake to find a package configuration file provided by "OpenJPEG",
      but CMake did not find one.

      Could not find a package configuration file provided by "OpenJPEG" with any
      of the following names:

        OpenJPEGConfig.cmake
        openjpeg-config.cmake

      Add the installation prefix of "OpenJPEG" to CMAKE_PREFIX_PATH or set
      "OpenJPEG_DIR" to a directory containing one of the above files.  If
      "OpenJPEG" provides a separate development package or SDK, be sure it has
      been installed.

    CMake Error at CMakeLists.txt:255 (message):
      Install libopenjpeg2 before trying to build poppler.  You can also decide
      to use the internal unmaintained JPX decoder or none at all.

    -- Configuring incomplete, errors occurred!
    See also "/tmp/poppler/build/CMakeFiles/CMakeOutput.log".
    See also "/tmp/poppler/build/CMakeFiles/CMakeError.log".

To figure out what exactly to do now, I have to read some
CMakeLists.txt. This commit improves the situation by producing the
following output instead:

    -- Found Iconv: /usr/lib/x86_64-linux-gnu/libc.so
    CMake Warning at CMakeLists.txt:254 (find_package):
      By not providing "FindOpenJPEG.cmake" in CMAKE_MODULE_PATH this project has
      asked CMake to find a package configuration file provided by "OpenJPEG",
      but CMake did not find one.

      Could not find a package configuration file provided by "OpenJPEG" with any
      of the following names:

        OpenJPEGConfig.cmake
        openjpeg-config.cmake

      Add the installation prefix of "OpenJPEG" to CMAKE_PREFIX_PATH or set
      "OpenJPEG_DIR" to a directory containing one of the above files.  If
      "OpenJPEG" provides a separate development package or SDK, be sure it has
      been installed.

    -- Could NOT find openjpeg2.
    CMake Error at CMakeLists.txt:258 (message):
      Install libopenjpeg2 before trying to build poppler.  You can also decide
      to use the internal unmaintained JPX decoder or none at all.

      Possible options are: -DENABLE_LIBOPENJPEG=openjpeg2,
      -DENABLE_LIBOPENJPEG=none, -DENABLE_LIBOPENJPEG=unmaintained,

    -- Configuring incomplete, errors occurred!
    See also "/tmp/poppler/build/CMakeFiles/CMakeOutput.log".
    See also "/tmp/poppler/build/CMakeFiles/CMakeError.log".

Signed-off-by: Uli Schlachter <psychon@znc.in>