Warning in metis

[Frank Dellaert]

I get

[ 16%] Building C object gtsam/3rdparty/metis/libmetis/CMakeFiles/metis.dir/balance.c.o
/Users/dellaert/git/gtsam/gtsam/3rdparty/metis/libmetis/balance.c:23:9: warning: using integer absolute value
      function 'abs' when argument is of floating point type [-Wabsolute-value]
    if (iabs(ntpwgts[0]*graph->tvwgt[0]-graph->pwgts[0]) < 3*graph->tvwgt[0]/graph->nvtxs)
        ^
/Users/dellaert/git/gtsam/gtsam/3rdparty/metis/include/metis.h:110:25: note: expanded from macro 'iabs'
  #define iabs          abs
                        ^
/Users/dellaert/git/gtsam/gtsam/3rdparty/metis/libmetis/balance.c:23:9: note: use function 'fabsf' instead
    if (iabs(ntpwgts[0]*graph->tvwgt[0]-graph->pwgts[0]) < 3*graph->tvwgt[0]/graph->nvtxs)
        ^~~~
        fabsf
/Users/dellaert/git/gtsam/gtsam/3rdparty/metis/include/metis.h:110:25: note: expanded from macro 'iabs'
  #define iabs          abs
                        ^

Would be nice if we could use cmake to turn off this warning for metis only, but without modifying any of the metis files.