meson: link against libatomic if required

Some platforms like 32-Bit PowerPC need linking against libatomic.
Try to compile and link a very simple snippet of code which uses atomics
and make libatomic required if it fails.
Because we're using `dependency('atomic')`, the required meson versions
gets bumped to 1.7.0. See https://mesonbuild.com/Dependencies.html#atomic-stdatomic

Change-Id: I6a6a696471e1d352fb161c537ba9023b97c2d31e

lix/libutil/meson.build

@@ -333,6 +333,7 @@ dependencies = [
   libarchive,
   nlohmann_json,
   openssl,
+  libatomic,
 ]
 
 libutil_temp = library(
@@ -409,6 +410,7 @@ liblixutil = declare_dependency(
     kj,
     libarchive,
     nlohmann_json,
+    libatomic,
   ],
   link_with : libutil
 )

lix/nix/meson.build

@@ -178,6 +178,7 @@ nix = executable(
     capnp_rpc,
     nlohmann_json,
     kj,
+    libatomic,
   ],
   cpp_pch : cpp_pch,
   install : true,

meson.build

@@ -34,7 +34,7 @@
 
 project('lix', 'cpp', 'rust',
   # NOTE: when bumping this, remember to also bump meson_version in subprojects.
-  meson_version : '>=1.5.0',
+  meson_version : '>=1.7.0',
   version : run_command('bash', '-c', 'echo -n $(jq -r .version < ./version.json)$VERSION_SUFFIX', check : true).stdout().strip(),
   default_options : [
     'cpp_std=c++23',
@@ -380,6 +380,26 @@ if is_freebsd
   libprocstat = declare_dependency(link_args : [ '-lprocstat' ])
 endif
 
+libatomic_test_program = '''
+#include <atomic>
+int main() {
+  std::atomic<uint8_t> w1;
+  std::atomic<uint16_t> w2;
+  std::atomic<uint32_t> w4;
+  std::atomic<uint64_t> w8;
+  return ++w1 + ++w2 + ++w4 + ++w8;
+}
+'''
+
+libatomic = dependency('atomic', required : false)
+
+# Some platforms like 32-Bit PowerPC need libatomic because they're lacking 64-Bit hardware atomic instructions
+# and compilers don't handle this automatically (yet).
+# See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81358 and https://clang.llvm.org/docs/Toolchain.html#atomics-library
+if cxx.links(libatomic_test_program, name : 'test if simple atomic program links')
+  libatomic = dependency('atomic', required : true)
+endif
+
 #
 # Build-time tools
 #

subprojects/lix-clang-tidy/meson.build

@@ -1,5 +1,5 @@
 project('lix-clang-tidy', ['cpp', 'c'],
-  meson_version : '>=1.5.0',
+  meson_version : '>=1.7.0',
   version : '0.1',
   default_options : ['warning_level=3', 'cpp_std=c++20']
 )