Quote possibly empty variable to avoid error message

On RedHat 6.4 workstation the module doesn't find the necessary system
information, and gives an error message when the last parameter to the
string function is not present.
Read /etc/system-release if others not parseable
Corrected mismatched if-else-endif arguments
Merge pull request #29 from rolk/029_distrib

Fix bug in reporting of system release (on RHEL 6.4)
Probe for library without using STL

If dune-grid has been compiled with _GLIBCXX_DEBUG, linking the test
program will fail since probing doesn't get the flags that are specified
for the main project, and thus configuration halts. Instead, we use a
smaller test program which doesn't need proper STL linkage to compile
and link.
Merge pull request #31 from rolk/031_stlcheck

Probe for library without using STL
Initialize MPIHelper before testing to fix issue #32.
Merge pull request #33 from blattms/issue-32

Initialize MPIHelper before testing to fix issue #32.
Change marker EXCLUDE_ALL to EXCLUDE_FROM_ALL in opm_compile_satellite

Previously, when passing EXCLUDE_ALL to opm_compile_satellite this
resulted a cmake error:

CMake Error at cmake/Modules/UseDebugSymbols.cmake:71 (get_target_property):
  Cannot find source file:

    EXCLUDE_ALL

  Tried extensions .c .C .c++ .cc .cpp .cxx .m .M .mm .h .hh .h++ .hm .hpp
  .hxx .in .txx
Call Stack (most recent call first):
  cmake/Modules/OpmSatellites.cmake:77 (strip_debug_symbols)
  CMakeLists.txt:176 (opm_compile_satellites)
Changed std::tr1 occurences to boost.

std::tr1 might not be supported by all compilers and will eventually
be dropped by others. Using boost instead makes this more
portable.
Merge pull request #35 from blattms/non-tr1

Changed std::tr1 occurences to boost.
Adjusts macro documentation to recent change.
Merge pull request #34 from blattms/satellite-exclude-all

Change marker EXCLUDE_ALL to EXCLUDE_FROM_ALL in opm_compile_satellite
Fixes the compilation of most not-unit tests and adds build directives for them.

This patch adds the target not_unit_tests for building most of the
non-unit test of the module to the CMake build system. The following
non-unit tests will be build:
check_grid_normals
buildcpgrid_test
max_zdist_test
finitevolume
mapper_test
This patch also includes various fixes for the above tests to make them
compile.
partition_test.cpp
Finished renaming NON_UNIT_TESTS to NOT_UNIT_TEST.

I missed one occasion previously. As a result not tests
were built. This patch fixes this.
Merge pull request #36 from blattms/fix-builds-of-some-non-unit-test

Fixes the compilation of and add build directives for most not-unit tests
Removes not-unit tests from targets to be run during make test
Merge pull request #37 from blattms/do-not-run-non-unit-tests

Removes not-unit tests from targets to be run during make test
check for the C++-2011 'auto' keyword.

It seems like the corresponding FindCXX11Features.cmake file
was not updated for this module.
make the c++-2011 feature set of GCC 4.4 mandatory

we now produce an error even if the module in question does not
actually use some of these features.