configure: add -Wstrict-prototypes when available

GCC (and Clang) have this useful warning that is not enabled by -Wall or
-Wextra. This will ensure that issues like those fixed in
4da52e3630
will trigger warnings.

Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

configure

@@ -5637,6 +5637,7 @@ disabled iconv || check_func_headers iconv.h iconv || check_lib2 iconv.h iconv -
 enabled debug && add_cflags -g"$debuglevel" && add_asflags -g"$debuglevel"
 
 # add some useful compiler flags if supported
+check_cflags -Wstrict-prototypes
 check_cflags -Wdeclaration-after-statement
 check_cflags -Wall
 check_cflags -Wdisabled-optimization