zimcheck/meson.build: Fix FreeBSD build; Avoid crushing 'thread_dep'

On FreeBSD, LLVM's libc++ also relies on pthread.

zimcheck's 'meson.build' would use the same 'thread_dep' variable set in the
root 'meson.build' file on building zimwriterfs, crushing the value set there in
some cases, which would then be used in zimwriterfs' 'meson.build'.

Note: Some of the introduced line is very long but has not been wrapped as Meson
0.49.2 is still used in the MacOS CI and it does not not support continuing
lines with '\'.

src/zimcheck/meson.build

@@ -8,11 +8,12 @@ endif
 
 inc = include_directories(extra_include)
 
-if compiler.get_id() == 'gcc' and host_machine.system() == 'linux'
-  # C++ std::thread is implemented using pthread on linux by gcc
-  thread_dep = dependency('threads')
-else
-  thread_dep = dependency('', required:false)
+zimcheck_deps = [libzim_dep, icu_dep, docopt_dep]
+
+# C++ std::thread is implemented using pthread on Linux by GCC, and on FreeBSD
+# for both GCC and LLVM.
+if (host_machine.system() == 'linux' and compiler.get_id() == 'gcc') or host_machine.system() == 'freebsd'
+  zimcheck_deps += dependency('threads')
 endif
 
 executable('zimcheck',
@@ -23,7 +24,5 @@ executable('zimcheck',
   '../tools.cpp',
   '../metadata.cpp',
   include_directories : inc,
-  dependencies: [libzim_dep, icu_dep, thread_dep, docopt_dep],
+  dependencies: zimcheck_deps,
   install: true)
-
-