introduce operating system version ranges as part of the target

 * re-introduce `std.build.Target` which is distinct from `std.Target`.
   `std.build.Target` wraps `std.Target` so that it can be annotated as
   "the native target" or an explicitly specified target.
 * `std.Target.Os` is moved to `std.Target.Os.Tag`. The former is now a
   struct which has the tag as well as version range information.
 * `std.elf` gains some more ELF header constants.
 * `std.Target.parse` gains the ability to parse operating system
   version ranges as well as glibc version.
 * Added `std.Target.isGnuLibC()`.
 * self-hosted dynamic linker detection and glibc version detection.
   This also adds the improved logic using `/usr/bin/env` rather than
   invoking the system C compiler to find the dynamic linker when zig
   is statically linked. Related: #2084
   Note: this `/usr/bin/env` code is work-in-progress.
 * `-target-glibc` CLI option is removed in favor of the new `-target`
   syntax. Example: `-target x86_64-linux-gnu.2.27`

closes #1907

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/std/dynamic_library.zig b/lib/std/dynamic_library.zig
index 409dace20f..34f45894fb 100644
--- a/lib/std/dynamic_library.zig
+++ b/lib/std/dynamic_library.zig
@@ -11,7 +11,7 @@ const system = std.os.system;
 const maxInt = std.math.maxInt;
 const max = std.math.max;
 
-pub const DynLib = switch (builtin.os) {
+pub const DynLib = switch (builtin.os.tag) {
     .linux => if (builtin.link_libc) DlDynlib else ElfDynLib,
     .windows => WindowsDynLib,
     .macosx, .tvos, .watchos, .ios, .freebsd => DlDynlib,
@@ -390,7 +390,7 @@ pub const DlDynlib = struct {
 };
 
 test "dynamic_library" {
-    const libname = switch (builtin.os) {
+    const libname = switch (builtin.os.tag) {
         .linux, .freebsd => "invalid_so.so",
         .windows => "invalid_dll.dll",
         .macosx, .tvos, .watchos, .ios => "invalid_dylib.dylib",