build: add support for Swift toolchains not provided by Apple

the xcode-select tool only properly works with Apple provided toolchains
but not with third party ones from swift.org. in the latter case the
swift compiler executable is found in the proper path but the swift libs
from the xcode or command line tools will be picked. this leads to a
not wanted discrepancy of the swift compiler and libs and possible
errors.

instead of relying on the xcode-select tool search for the libs relative
to the swift executable. that relative path seems to be the same for all
toolchains. if for any reasons a swift executable is not found in the
relative path, fall back to the old xcode-select method.

furthermore, both static and dynamic libs will be searched for but only
the former will be used for now. this is a preparation for the upcoming
swift 5 migration.

waftools/detections/compiler_swift.py

@@ -1,4 +1,5 @@
 import re
+import os.path
 from waflib import Utils
 from distutils.version import StrictVersion
 
@@ -36,20 +37,55 @@ def __add_swift_library_linking_flags(ctx, swift_library):
     ])
 
 def __find_swift_library(ctx):
-    swift_library_paths = [
-        'Toolchains/XcodeDefault.xctoolchain/usr/lib/swift_static/macosx',
-        'usr/lib/swift_static/macosx'
-    ]
+    swift_libraries = {}
+    #first search for swift libs relative to the swift compiler executable
+    swift_library_relative_paths = {
+        'SWIFT_LIB_DYNAMIC': '../../lib/swift/macosx',
+        'SWIFT_LIB_STATIC': '../../lib/swift_static/macosx'
+    }
+
+    for lib_type, path in swift_library_relative_paths.items():
+        lib_path = os.path.join(ctx.env.SWIFT, path)
+        swift_library = ctx.root.find_dir(lib_path)
+        if swift_library is not None:
+            swift_libraries[lib_type] = swift_library.abspath()
+
+    #fall back to xcode-select path
+    swift_library_paths = {
+        'SWIFT_LIB_DYNAMIC': [
+            'Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/macosx',
+            'usr/lib/swift/macosx'
+        ],
+        'SWIFT_LIB_STATIC': [
+            'Toolchains/XcodeDefault.xctoolchain/usr/lib/swift_static/macosx',
+            'usr/lib/swift_static/macosx'
+        ]
+    }
     dev_path = __run(['xcode-select', '-p'])[1:]
 
-    ctx.start_msg('Checking for Swift Library')
-    for path in swift_library_paths:
-        swift_library = ctx.root.find_dir([dev_path, path])
-        if swift_library is not None:
-            ctx.end_msg(swift_library.abspath())
-            __add_swift_library_linking_flags(ctx, swift_library.abspath())
-            return
-    ctx.end_msg(False)
+    for lib_type, paths in swift_library_paths.items():
+        for path in paths:
+            if lib_type not in swift_libraries:
+                swift_library = ctx.root.find_dir([dev_path, path])
+                if swift_library is not None:
+                    swift_libraries[lib_type] = swift_library.abspath()
+                    break
+            else:
+                break
+
+    #check if library paths were found
+    ctx.start_msg('Checking for dynamic Swift Library')
+    if 'SWIFT_LIB_DYNAMIC' in swift_libraries:
+        ctx.end_msg(swift_libraries['SWIFT_LIB_DYNAMIC'])
+    else:
+        ctx.end_msg(False)
+
+    ctx.start_msg('Checking for static Swift Library')
+    if 'SWIFT_LIB_STATIC' in swift_libraries:
+        ctx.end_msg(swift_libraries['SWIFT_LIB_STATIC'])
+        __add_swift_library_linking_flags(ctx, swift_libraries['SWIFT_LIB_STATIC'])
+    else:
+        ctx.end_msg(False)
 
 def __find_macos_sdk(ctx):
     ctx.start_msg('Checking for macOS SDK')