[libcxx] Redo adding support for building and testing with an ABI library not along linker paths

Summary:
This is the second attempt at allowing for the use of libraries that the linker cannot find. The first attempt used `CMAKE_LIBRARY_PATH` and `find_library` to select which ABI library should be used. There were a number of problems with this approach:

- `find_library` didn't work with cmake targets (ie in-tree libcxxabi build)
- It wasn't always possible to determine where `find_library` actually found your library.
- `target_link_libraries` inserted the path of the ABI library into libc++'s RPATH when `find_library` was used.
- Linking libc++ and it's ABI library is a special case. It's a lot easier to keep it simple. 

After discussion with @cbergstrum a new approach was decided upon.
This patch achieve the same ends by simply using `LIBCXX_CXX_ABI_LIBRARY_PATH` to specify where to find the library (if the linker won't find it). When this variable is defined it is simply added as a library search path when linking libc++. It is a lot easier to duplicate this behavior in LIT. It also prevents libc++ from being linked with an RPATH.






Reviewers: mclow.lists, cbergstrom, chandlerc, danalbert

Reviewed By: chandlerc, danalbert

Subscribers: chandlerc, cfe-commits

Differential Revision: http://reviews.llvm.org/D5860

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@220157 91177308-0d34-0410-b5e6-96231b3b80d8

test/lit.cfg

@@ -374,10 +374,13 @@ class Configuration(object):
 
     def configure_link_flags(self):
         # Configure library search paths
-        lpaths = self.get_lit_conf('library_paths', '').split(';')
-        lpaths = [l for l in lpaths if l.strip()]
+        abi_library_path = self.get_lit_conf('abi_library_path', '')
         self.link_flags += ['-L' + self.obj_root + '/lib']
-        self.link_flags += ['-L' + l for l in lpaths]
+        if not self.use_system_lib:
+            self.link_flags += ['-Wl,-rpath', '-Wl,' + self.obj_root + '/lib']
+        if abi_library_path:
+            self.link_flags += ['-L' + abi_library_path,
+                                '-Wl,-rpath', '-Wl,' + abi_library_path]
         # Configure libraries
         self.link_flags += ['-lc++']
         link_flags_str = self.get_lit_conf('link_flags')
@@ -412,11 +415,6 @@ class Configuration(object):
         if link_flags_str:
             self.link_flags += shlex.split(link_flags_str)
 
-        # Configure library runtime search paths
-        if not self.use_system_lib:
-            self.link_flags += ['-Wl,-rpath', '-Wl,' + self.obj_root + '/lib']
-        for l in lpaths:
-            self.link_flags += ['-Wl,-rpath', '-Wl,' + l]
 
     def configure_std_flag(self):
         # Try and get the std version from the command line. Fall back to

test/lit.site.cfg.in

@@ -7,7 +7,7 @@ config.python_executable     = "@PYTHON_EXECUTABLE@"
 config.enable_shared         = @LIBCXX_ENABLE_SHARED@
 config.cxx_abi               = "@LIBCXX_CXX_ABI_LIBNAME@"
 config.llvm_use_sanitizer    = "@LLVM_USE_SANITIZER@"
-config.library_paths         = "@CMAKE_LIBRARY_PATH@"
+config.abi_library_path      = "@LIBCXX_CXX_ABI_LIBRARY_PATH@"
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@LIBCXX_SOURCE_DIR@/test/lit.cfg")