configure: add further -Werror=... options to detected CFLAGS

these four warning options were overlooked previously, likely because
they're not part of GCC's -Wall. they all detect constraint violations
(invalid C at the source level) and should always be on in -Werror
form.

configure

@@ -522,6 +522,10 @@ tryflag CFLAGS_AUTO -Werror=implicit-function-declaration
 tryflag CFLAGS_AUTO -Werror=implicit-int
 tryflag CFLAGS_AUTO -Werror=pointer-sign
 tryflag CFLAGS_AUTO -Werror=pointer-arith
+tryflag CFLAGS_AUTO -Werror=int-conversion
+tryflag CFLAGS_AUTO -Werror=incompatible-pointer-types
+tryflag CFLAGS_AUTO -Werror=discarded-qualifiers
+tryflag CFLAGS_AUTO -Werror=discarded-array-qualifiers
 
 #
 # GCC ignores unused arguements by default, but Clang needs this extra