allow custom install prefix

--- a/utils/build-presets.ini	2019-04-11 14:51:40.060259462 +0200
+++ b/utils/build-presets.ini	2019-04-11 15:16:17.471137969 +0200
@@ -728,7 +728,7 @@
 install-swiftpm
 install-xctest
 install-libicu
-install-prefix=/usr
+install-prefix=%(install_prefix)s
 swift-install-components=autolink-driver;compiler;clang-resource-dir-symlink;stdlib;swift-remote-mirror;sdk-overlay;license;sourcekit-inproc
 llvm-install-components=llvm-cov;llvm-profdata;IndexStore;clang;clang-headers;compiler-rt
 install-libcxx