configure: test for -Wfloat-conversion

supported by clang, gcc-4.9+

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

configure

@@ -586,6 +586,7 @@ process_toolchain() {
         check_add_cflags -Wall
         check_add_cflags -Wdeclaration-after-statement
         check_add_cflags -Wdisabled-optimization
+        check_add_cflags -Wfloat-conversion
         check_add_cflags -Wpointer-arith
         check_add_cflags -Wtype-limits
         check_add_cflags -Wcast-qual