Missing link to atomic on some architectures

[legoktm]

On Debian, builds for libkiwix are failing on armel, mipsel, and a few other architectures because of a missing link to atomic.

The error is something like:

/usr/bin/ld: src/25a6634@@kiwix@sha/server_request_context.cpp.o: in function `std::__atomic_base<unsigned long long>::fetch_add(unsigned long long, std::memory_order)':
/usr/include/c++/9/bits/atomic_base.h:541: undefined reference to `__atomic_fetch_add_8'
collect2: error: ld returned 1 exit status

Here are the full build logs: https://buildd.debian.org/status/package.php?p=libkiwix&suite=experimental

Searching around a bit, it seems we just need to add -latomic for those architectures. See e.g. how redis fixed this in redis/redis#6275 / redis/redis@f5d4853

For meson I think we can use https://mesonbuild.com/Reference-manual.html#build_machine-object to vary what we link against.

[legoktm]

The patch I have so far is:

From: Kunal Mehta <legoktm@debian.org>
Date: Sun, 21 Jun 2020 01:02:00 -0700
Subject: Pass -latomic for architectures that need it

---
 meson.build | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/meson.build b/meson.build
index 864b46a..e54a3a5 100644
--- a/meson.build
+++ b/meson.build
@@ -18,6 +18,11 @@ if 'java' in wrapper
   add_languages('java')
 endif
 
+# See https://github.com/kiwix/kiwix-lib/issues/371
+if target_machine.cpu() in ['arm', 'mips', 'm68k', 'ppc']
+  extra_libs += '-latomic'
+endif
+
 thread_dep = dependency('threads')
 libicu_dep = dependency('icu-i18n', static:static_deps)
 libzim_dep = dependency('libzim', version : '>=6.1.1', static:static_deps)

I'm just waiting to be able to log into Debian's porterboxes to test building on those architectures.