fails to build on Arch Linux

As far as I can tell, `-Wunsigned-char` is invalid for `gcc`.

```
| Syntax error
configure:6911: gcc -E -Wunsigned-char conftest.c
gcc: error: unrecognized command-line option '-Wunsigned-char'; did you mean '-funsigned-char'?
configure:6911: $? = 1
```

Changing this to `-funsigned-char` as suggested above, does get past the above error/failure, but the build then fails with the following:

```
make[2]: Entering directory '/tmp/makepkg/resource-agents/src/resource-agents-4.10.0/heartbeat'
CC send_ua.o
CC IPv6addr_utils.o
CCLD send_ua
make[2]: Leaving directory '/tmp/makepkg/resource-agents/src/resource-agents-4.10.0/heartbeat'
Making all in tools
make[2]: Entering directory '/tmp/makepkg/resource-agents/src/resource-agents-4.10.0/tools'
Making all in ocft
make[3]: Entering directory '/tmp/makepkg/resource-agents/src/resource-agents-4.10.0/tools/ocft'
make[3]: Nothing to be done for 'all'.
make[3]: Leaving directory '/tmp/makepkg/resource-agents/src/resource-agents-4.10.0/tools/ocft'
make[3]: Entering directory '/tmp/makepkg/resource-agents/src/resource-agents-4.10.0/tools'
CC findif.o
CC storage_mon.o
CC send_arp-send_arp.linux.o
CC tickle_tcp.o
In file included from /usr/include/netinet/ip.h:24,
from tickle_tcp.c:28:
In function 'uint16_checksum',
inlined from 'tcp_checksum6' at tickle_tcp.c:101:9,
inlined from 'send_tickle_ack' at tickle_tcp.c:292:25:
tickle_tcp.c:59:40: error: 'phdr' may be used uninitialized [-Werror=maybe-uninitialized]
59 | sum += (uint32_t)ntohs(*data);
| ^~~~~
tickle_tcp.c: In function 'send_tickle_ack':
tickle_tcp.c:90:18: note: 'phdr' declared here
90 | uint32_t phdr[2];
| ^~~~
cc1: all warnings being treated as errors
CCLD storage_mon
make[3]: *** [Makefile:755: tickle_tcp.o] Error 1
make[3]: *** Waiting for unfinished jobs....
make[3]: Leaving directory '/tmp/makepkg/resource-agents/src/resource-agents-4.10.0/tools'
make[2]: *** [Makefile:928: all-recursive] Error 1
make[2]: Leaving directory '/tmp/makepkg/resource-agents/src/resource-agents-4.10.0/tools'
make[1]: *** [Makefile:517: all-recursive] Error 1
make[1]: Leaving directory '/tmp/makepkg/resource-agents/src/resource-agents-4.10.0'
make: *** [Makefile:450: all] Error 2
==> ERROR: A failure occurred in build().
Aborting...
```