configure: Enable -Wno-implicit-const-int-float-conversion if available

This silences a lot of compile warnings (around 160 instances at least), when
compiling with Clang.

These warnings look like this:

    libavformat/http.c:176:133: warning: implicit conversion from 'long long' to 'double' changes value from 9223372036854775807 to 9223372036854775808 [-Wimplicit-const-int-float-conversion]
      176 |     { "end_offset", "try to limit the request to bytes preceding this offset", OFFSET(end_off), AV_OPT_TYPE_INT64, { .i64 = 0 }, 0, INT64_MAX, D },
          |     ~                                                                                                                               ^~~~~~~~~

Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -7460,6 +7460,7 @@ check_disable_warning -Wno-pointer-sign
 check_disable_warning -Wno-unused-const-variable
 check_disable_warning -Wno-bool-operation
 check_disable_warning -Wno-char-subscripts
+check_disable_warning -Wno-implicit-const-int-float-conversion
 
 check_disable_warning_headers(){
     warning_flag=-W${1#-Wno-}