build-sys: turn some warnings into errors

Make the build sys error out on missing function prototypes, missing
variable declarations, implicit function declarations or forgotten return
statements.

None of these conditions are acceptable, and by making them hard errors, the
build bots can detect them earlier.

configure.ac

@@ -159,10 +159,12 @@ CC_CHECK_FLAGS_APPEND([with_cflags], [CFLAGS], [\
         -Wdeclaration-after-statement \
         -Wfloat-equal \
         -Wsuggest-attribute=noreturn \
-        -Wmissing-prototypes \
+        -Werror=missing-prototypes \
+        -Werror=implicit-function-declaration \
+        -Werror=missing-declarations \
+        -Werror=return-type \
         -Wstrict-prototypes \
         -Wredundant-decls \
-        -Wmissing-declarations \
         -Wmissing-noreturn \
         -Wshadow \
         -Wendif-labels \