meson: add variable check for mutex usage

meson.build

@@ -1,9 +1,15 @@
 inc = include_directories('.')
 
+cpp_args = []
+
+if get_variable('log_thread_safe', false)
+    cpp_args += '-DLOG_MUTEX=1'
+endif
+
 lib_okaestne_log = static_library('okaestne-log',
     'Log.cpp',
     include_directories: inc,
-    cpp_args: ['-O2', '-std=c++11', '-Wall', '-Wextra', '-pedantic-errors', '-g']
+    cpp_args: cpp_args
 )
 
 # to depend on in other projects: