configure: test for -Wfloat-conversion
configure: test for -Wfloat-conversion

supported by clang, gcc-4.9+

Change-Id: I893766de7307fef9a8b68c0cfae137c9d3b0dbe8
(cherry picked from commit 889ed5b158fc280927f2de9172d48245c3b735a7)

GrokImageCompression/aom,smarter/aom,mbebenita/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,mbebenita/aom,smarter/aom,mbebenita/aom,luctrudeau/aom,GrokImageCompression/aom,GrokImageCompression/aom,luctrudeau/aom,mbebenita/aom,GrokImageCompression/aom,mbebenita/aom,luctrudeau/aom,mbebenita/aom,mbebenita/aom,GrokImageCompression/aom,luctrudeau/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,luctrudeau/aom