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, 15 insertions, 10 deletions

diff --git a/lib/std/elf.zig b/lib/std/elf.zig
index 9e4c1ac5f6..007a01bb90 100644
--- a/lib/std/elf.zig
+++ b/lib/std/elf.zig
@@ -349,16 +349,6 @@ pub const Elf = struct {
     program_headers: []ProgramHeader,
     allocator: *mem.Allocator,
 
-    /// Call close when done.
-    pub fn openPath(allocator: *mem.Allocator, path: []const u8) !Elf {
-        @compileError("TODO implement");
-    }
-
-    /// Call close when done.
-    pub fn openFile(allocator: *mem.Allocator, file: File) !Elf {
-        @compileError("TODO implement");
-    }
-
     pub fn openStream(
         allocator: *mem.Allocator,
         seekable_stream: *io.SeekableStream(anyerror, anyerror),
@@ -554,6 +544,21 @@ pub const Elf = struct {
 };
 
 pub const EI_NIDENT = 16;
+
+pub const EI_CLASS = 4;
+pub const ELFCLASSNONE = 0;
+pub const ELFCLASS32 = 1;
+pub const ELFCLASS64 = 2;
+pub const ELFCLASSNUM = 3;
+
+pub const EI_DATA = 5;
+pub const ELFDATANONE = 0;
+pub const ELFDATA2LSB = 1;
+pub const ELFDATA2MSB = 2;
+pub const ELFDATANUM = 3;
+
+pub const EI_VERSION = 6;
+
 pub const Elf32_Half = u16;
 pub const Elf64_Half = u16;
 pub const Elf32_Word = u32;