build: add -Wno-format-signedness

gcc5 introduced this option (gcc4 silently ignores it, which is fine).
Given that gcc5 thinks 'unsigned char'/'unsigned short' is promoted to
'int' for var-args, stuff like this spits out warnings:
    uint8_t x;
    printf("%" PRIu8", x);

gcc5 promots 'x' to 'int', instead of 'unsigned int' and thus gets a
signedness-warnings as it expects an 'unsigned int'.

glibc states otherwise: unsigneds are always promoted to 'unsigned int'.
Until gcc and glibc figure this out, lets just ignore that warning (which
is totally useless in its current form).

configure.ac

@@ -186,6 +186,7 @@ CC_CHECK_FLAGS_APPEND([with_cflags], [CFLAGS], [\
         -Wno-unused-parameter \
         -Wno-missing-field-initializers \
         -Wno-unused-result \
+        -Wno-format-signedness \
         -Werror=overflow \
         -Wdate-time \
         -Wnested-externs \