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.

Change-Id: I6a6a696471e1d352fb161c537ba9023b97c2d31e
(cherry picked from commit 95448347ea)

lix/libutil/meson.build

@@ -306,6 +306,7 @@ libutil = library(
     openssl,
     nlohmann_json,
     kj,
+    libatomic,
   ],
   include_directories : [ '../..' ],
   cpp_pch : cpp_pch,
@@ -343,6 +344,7 @@ liblixutil = declare_dependency(
     # lix-base pkg-config externally)
     kj,
     libarchive,
+    libatomic,
   ],
   link_with : libutil
 )

lix/nix/meson.build

@@ -177,6 +177,7 @@ nix = executable(
     boehm,
     nlohmann_json,
     kj,
+    libatomic,
   ],
   cpp_pch : cpp_pch,
   install : true,

meson.build

@@ -378,6 +378,26 @@ 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 = cxx.find_library('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 not cxx.links(libatomic_test_program, name : 'test if simple atomic program links')
+  libatomic = cxx.find_library('atomic', required : true)
+endif
+
 #
 # Build-time tools
 #