[build-script] Set INSTALL_PREFIX in Python

Rather than setting a default value for the INSTALL_PREFIX in the
build-script-impl shellscript, do so in the Python build-script. A small step
towards achieving SR-237.

Author: modocache

utils/build-script

@@ -532,6 +532,11 @@ also build for Apple watchos, but disallow tests that require an watchOS device"
 name of the directory under $SWIFT_BUILD_ROOT where the build products will be
 placed""",
         metavar="PATH")
+    parser.add_argument("--install-prefix",
+        help="The installation prefix. This is where built Swift products "
+             "(like bin, lib, and include) will be installed.",
+        metavar="PATH",
+        default=swift_build_support.targets.install_prefix())
 
     parser.add_argument("-j", "--jobs",
         help="""
@@ -566,6 +571,7 @@ the number of parallel build jobs to use""",
         '--host-target',
         '--skip-build',
         '--show-sdks',
+        '--install-prefix',
     ]))
 
     if args.host_target is None:
@@ -830,6 +836,7 @@ the number of parallel build jobs to use""",
     build_script_impl_args = [
         os.path.join(SWIFT_SOURCE_ROOT, "swift", "utils", "build-script-impl"),
         "--build-dir", build_dir,
+        "--install-prefix", os.path.abspath(args.install_prefix),
         "--host-target", args.host_target,
         "--host-cc", host_clang.cc,
         "--host-cxx", host_clang.cxx,

utils/build-script-impl

@@ -718,14 +718,6 @@ function true_false() {
 # Set default values for command-line parameters.
 #
 
-if [[ "${INSTALL_PREFIX}" == "" ]] ; then
-    if [[ "$(uname -s)" == "Darwin" ]] ; then
-        INSTALL_PREFIX="/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr"
-    else
-        INSTALL_PREFIX="/usr"
-    fi
-fi
-
 if [[ "$(uname -s)" == "Darwin" ]] ; then
     TOOLCHAIN_PREFIX=$(echo ${INSTALL_PREFIX} | sed -E 's/\/usr$//')
 fi

utils/swift_build_support/swift_build_support/targets.py

@@ -44,3 +44,15 @@ def host_target():
             return 'freebsd-x86_64'
 
     return None
+
+
+def install_prefix():
+    """
+    Returns the default path at which built Swift products (like bin, lib,
+    and include) will be installed, based on the host machine's operating
+    system.
+    """
+    if platform.system() == 'Darwin':
+        return '/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr'
+    else:
+        return '/usr'