compilers: cpp: fix header name and return value use in header check

There are two issues:

1. has_header() wants just the header name without surrounding <>
or similar; it fails otherwise.

2. has_header() returns a tuple of two bools, where the first element
determines whether or not the header has been found. So use
that element specifically, otherwise the tuple will always evaluate
to true because it is not empty.

Fixes: 675b47b ("compilers: cpp: improve libc++ vs libstdc++ detection (again)")

Author: pobrn

mesonbuild/compilers/cpp.py

@@ -185,7 +185,7 @@ class _StdCPPLibMixin(CompilerMixinBase):
 
     def language_stdlib_provider(self, env: Environment) -> str:
         # https://stackoverflow.com/a/31658120
-        header = 'version' if self.has_header('<version>', '', env) else 'ciso646'
+        header = 'version' if self.has_header('version', '', env)[0] else 'ciso646'
         is_libcxx = self.has_header_symbol(header, '_LIBCPP_VERSION', '', env)[0]
         lib = 'c++' if is_libcxx else 'stdc++'
         return lib