configure.ac: fix two smaller issues
1.  If gcc-multilib pkg not present, configure will now test for this
    by compiling with -m32 with "#include <features.h>".  If compiling fails,
    we propose to the user to install gcc-multilib.  (We previously
    did this only for g++-multilib package.)
2.  AC_PROG_CPP added
    (Without this, any tests using the preprocessor were failing due to
     undefined $CPP.  Compiler ($CC) would then do the right thing,
     and we would see a WARNING on ./configure .)


git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2383 ba6dc0ae-232f-0410-883f-9ebac64fc565