added define to enable MADV_FREE usage on Linux
added define to enable MADV_FREE usage on Linux

Building with -DTCMALLOC_USE_MADV_FREE will enable usage of MADV_FREE on
Linux if glibc copy of kernel headers has MADV_FREE defined.

I.e. so that people can test this more easily.

Affects ticket #780.

alk/gperftools,acmorrow/gperftools,chencl/gperftools,alk/gperftools,romange/gperftools,rzinsly/gperftools,romange/gperftools,acmorrow/gperftools,gperftools/gperftools,rzinsly/gperftools,xiaoyur347/gperftools,acmorrow/gperftools,acmorrow/gperftools,xiaoyur347/gperftools,rzinsly/gperftools,alk/gperftools,gperftools/gperftools,gperftools/gperftools,xiaoyur347/gperftools,romange/gperftools,chencl/gperftools,xiaoyur347/gperftools,chencl/gperftools,rzinsly/gperftools,gperftools/gperftools,romange/gperftools,chencl/gperftools,alk/gperftools