Error on implicit function declarations

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

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22269 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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