compilers: make use of mlog.log_once

I'm sure there are other places that could use this, but I didn't see
any right off that bat.

mesonbuild/compilers/c.py

@@ -326,8 +326,6 @@ class IntelClCCompiler(IntelVisualStudioLikeCompiler, VisualStudioLikeCCompilerM
 
     """Intel "ICL" compiler abstraction."""
 
-    __have_warned = False
-
     def __init__(self, exelist, version, for_machine: MachineChoice,
                  is_cross, info: 'MachineInfo', exe_wrap, target, **kwargs):
         CCompiler.__init__(self, exelist, version, for_machine, is_cross,
@@ -346,9 +344,7 @@ class IntelClCCompiler(IntelVisualStudioLikeCompiler, VisualStudioLikeCCompilerM
         args = []
         std = options['c_std']
         if std.value == 'c89':
-            if not self.__have_warned:
+            mlog.warning("ICL doesn't explicitly implement c89, setting the standard to 'none', which is close.", once=True)
-                self.__have_warned = True
-                mlog.warning("ICL doesn't explicitly implement c89, setting the standard to 'none', which is close.")
         elif std.value != 'none':
             args.append('/Qstd:' + std.value)
         return args

mesonbuild/compilers/cpp.py

@@ -484,7 +484,7 @@ class CPP11AsCPP14Mixin:
         # if one is using anything before that point, one cannot set the standard.
         if options['cpp_std'].value in {'vc++11', 'c++11'}:
             mlog.warning(self.id, 'does not support C++11;',
-                         'attempting best effort; setting the standard to C++14')
+                         'attempting best effort; setting the standard to C++14', once=True)
             # Don't mutate anything we're going to change, we need to use
             # deepcopy since we're messing with members, and we can't simply
             # copy the members because the option proxy doesn't support it.