Auto merge of #28495 - semarie:stdcpp-name, r=alexcrichton

it makes rustc compatible with gcc installation that are using
`--program-transform-name' configure flag (on OpenBSD for example).

- detects at configure the name of stdc++ library on the system

- use the detected name in llvm makefile (with enable-static-stdcpp),
  and pass it to mklldeps.py

- generate mklldeps.rs using this detected name

note that CFG_STDCPP_NAME is about stdc++ name, not about libc++. If
using libc++, the default name will be `stdc++', but it won't be used
when linking.

r? @alexcrichton 

I added this support globally instead of just for OpenBSD as it isn't specially related to OpenBSD (except OpenBSD use it for gcc-4.9). And as I would had to change `llvm.mk', having just a default value in `configure' for others platforms won't be very useful.

src/etc/mklldeps.py

@@ -17,6 +17,7 @@ f = open(sys.argv[1], 'wb')
 components = sys.argv[2].split() # splits on whitespace
 enable_static = sys.argv[3]
 llvm_config = sys.argv[4]
+stdcpp_name = sys.argv[5]
 
 f.write("""// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
@@ -77,7 +78,7 @@ for lib in out.strip().split(' '):
 out = run([llvm_config, '--cxxflags'])
 if enable_static == '1':
     assert('stdlib=libc++' not in out)
-    f.write("#[link(name = \"stdc++\", kind = \"static\")]\n")
+    f.write("#[link(name = \"" + stdcpp_name + "\", kind = \"static\")]\n")
 else:
     # Note that we use `cfg_attr` here because on MSVC the C++ standard library
     # is not c++ or stdc++, but rather the linker takes care of linking the
@@ -85,7 +86,7 @@ else:
     if 'stdlib=libc++' in out:
         f.write("#[cfg_attr(not(target_env = \"msvc\"), link(name = \"c++\"))]\n")
     else:
-        f.write("#[cfg_attr(not(target_env = \"msvc\"), link(name = \"stdc++\"))]\n")
+        f.write("#[cfg_attr(not(target_env = \"msvc\"), link(name = \"" + stdcpp_name + "\"))]\n")
 
 # Attach everything to an extern block
 f.write("extern {}\n")