Better error message if lsb_release is missing

[peterfpeterson]

Joachim Wutke reported this error when lsb_release is missing (like on debian):

-- Found the following Boost libraries:
--   date_time
--   regex
CMake Warning (dev) at Build/CMake/LinuxSetup.cmake:4 (find_package):
   Syntax Warning in cmake code at

/home/jwu/g/othersw/mantid-3.0.0-Source/Build/CMake/FindPythonLibs.cmake:65:34

   Argument not separated from preceding token by whitespace.
Call Stack (most recent call first):
   Build/CMake/CommonSetup.cmake:222 (include)
   CMakeLists.txt:78 (include)
This warning is for project developers.  Use -Wno-dev to suppress it.

--  DIST:  CODENAME:
CMake Error at Build/CMake/LinuxSetup.cmake:93 (if):
   if given arguments:

     "STREQUAL" "RedHatEnterprise"

   Unknown arguments specified
Call Stack (most recent call first):
   Build/CMake/CommonSetup.cmake:222 (include)
   CMakeLists.txt:78 (include)


-- Configuring incomplete, errors occurred!
See also
"/home/jwu/g/othersw/mantid-3.0.0-Source/CMakeFiles/CMakeOutput.log".
See also
"/home/jwu/g/othersw/mantid-3.0.0-Source/CMakeFiles/CMakeError.log".

[peterfpeterson]

This issue was originally trac ticket 8501