compilers/cpp: check libc++ vs libstdc++ harder

Instead of hardcoding any values, hardcode what we think the most likely
implementation is, and check that first. It was pointed out that while
for example, Apple only provides libc++ and supports that for xcode, a
user could install a custom environment (such as homebrew) which uses
it's own copy of libstdc++, and we need to account for that. This means
that a library search will be done, but only once and the result will be
cached, on all systems.

mesonbuild/compilers/cpp.py

@@ -206,22 +206,19 @@ class _StdCPPLibMixin(CompilerMixinBase):
         machine = env.machines[self.for_machine]
         assert machine is not None, 'for mypy'
 
-        # We need to determine whether to us libc++ or libstdc++ In some cases
-        # we know the answer, so we'll hardcode those cases.  There are other
-        # cases where we can't know the answer just by looking at the OS, namely
-        # on Linux. In that case we have to fallback to manually checking
-        stdlib: str
+        # We need to determine whether to use libc++ or libstdc++. We can't
+        # really know the answer in most cases, only the most likely answer,
+        # because a user can install things themselves or build custom images.
+        search_order: T.List[str] = []
         if machine.system in {'android', 'darwin', 'dragonfly', 'freebsd', 'netbsd', 'openbsd'}:
-            stdlib = 'c++'
-        elif self.find_library('c++', env, []) is not None:
-            stdlib = 'c++'
-        elif self.find_library('stdc++', env, []) is not None:
-            stdlib = 'stdc++'
+            search_order = ['c++', 'stdc++']
         else:
-            # TODO: maybe a bug exception?
-            raise MesonException('Could not detect either libc++ or libstdc++ as your C++ stdlib implementation.')
-
-        return search_dirs + [f'-l{stdlib}']
+            search_order = ['stdc++', 'c++']
+        for lib in search_order:
+            if self.find_library(lib, env, []) is not None:
+                return search_dirs + [f'-l{lib}']
+        # TODO: maybe a bug exception?
+        raise MesonException('Could not detect either libc++ or libstdc++ as your C++ stdlib implementation.')
 
 
 class ClangCPPCompiler(_StdCPPLibMixin, ClangCompiler, CPPCompiler):