compiler: -Werror=return-type hardening

Warn whenever a function is defined with a return type that defaults to
int. Also warn about any return statement with no return value in a
function whose return type is not void (falling off the end of the
function body is considered returning without a value).

For C only, warn about a return statement with an expression in a
function whose return type is void, unless the expression type is also
void. As a GNU extension, the latter case is accepted without a warning
unless -Wpedantic is used.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

configure.ac

@@ -699,6 +699,7 @@ AX_CHECK_COMPILE_FLAG([-Wold-style-definition], [CFLAGS="$CFLAGS -Wold-style-def
 AX_CHECK_COMPILE_FLAG([-Winit-self], [CFLAGS="$CFLAGS -Winit-self"],,[-Werror])
 AX_CHECK_COMPILE_FLAG([-Wfloat-equal], [CFLAGS="$CFLAGS -Wfloat-equal"],,[-Werror])
 AX_CHECK_COMPILE_FLAG([-Wsuggest-attribute=noreturn], [CFLAGS="$CFLAGS -Wsuggest-attribute=noreturn"],,[-Werror])
+AX_CHECK_COMPILE_FLAG([-Werror=return-type], [CFLAGS="$CFLAGS -Werror=return-type"],,[-Werror])
 
 AX_CHECK_LINK_FLAG([-z relro], [LDLAGS="$LDLAGS -z relro"],,[])
 AX_CHECK_LINK_FLAG([-z now], [LDLAGS="$LDLAGS -z now"],,[])