Error on implicit function declarations

Turning on -Werror=implicit makes implicit function declarations
an error with supported compilers.

Originally committed as revision 22269 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2705,6 +2705,7 @@ check_cflags -Wwrite-strings
 check_cflags -Wtype-limits
 check_cflags -Wundef
 check_cflags -Wmissing-prototypes
+check_cflags -Werror=implicit
 enabled extra_warnings && check_cflags -Winline
 
 # add some linker flags