HPCC-9041 Quotes around R_INCLUDE_DIRS causing issue with cmake<2.8.8

Removed Quotes to allow cmake list to work correctly in all versions
of cmake >= 2.6.1.

Signed-off-by: Philip Schwartz <philip.schwartz@lexisnexis.com>
Merge pull request #4168 from pschwartz/HPCC-9041-fix-rembed-include-dirs-pre-cmake-2.8.8

HPCC-9041 Quotes around R_INCLUDE_DIRS causing issue with cmake<2.8.8

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>