MTCP: Undefine _FORTIFY_SOURCE in Makefile.

- If _FORTIFY_SOURCE is defined, compiler emits warning as MTCP is
  compiled without optimization.

git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2401 ba6dc0ae-232f-0410-883f-9ebac64fc565
Fix compiler warning for modify-env.c.

git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2402 ba6dc0ae-232f-0410-883f-9ebac64fc565