Fix libs3 compilation with g++ 7

There is some stricter bounds checking related to snprintf() calls with
the updated compiler, causing issues when building e.g. on Ubuntu
Bionic, see [0]. Add a flag to ignore that check.

[0] bji/libs3#75

libs3.patch

@@ -1,3 +1,16 @@
+diff --git a/GNUmakefile b/GNUmakefile
+index c81537c..ab1324e 100644
+--- a/GNUmakefile
++++ b/GNUmakefile
+@@ -136,7 +136,7 @@ ifndef CFLAGS
+ endif
+ endif
+
+-CFLAGS += -Wall -Werror -Wshadow -Wextra \
++CFLAGS += -Wall -Werror -Wshadow -Wextra -Wno-format-truncation \
+ -Iinc \
+ $(CURL_CFLAGS) $(LIBXML2_CFLAGS) \
+ -DLIBS3_VER_MAJOR=\"$(LIBS3_VER_MAJOR)\" \
 diff --git a/src/request.c b/src/request.c
 index 63174d0..090da42 100644
 --- a/src/request.c