std: allow overriding install path of artifacts

This is necessary when, for example, writing a PAM module which should
be installed to lib/security/module_name.so.

Currently the zig build system forces shared libraries to be prefixed
with "lib" and versions the so files as well as creating symlinks.
Furthermore, artifacts can currently only be installed to the bin or
lib directories, not sub-directories thereof.

Allowing the user to manually specify the install path allows maximum
flexibility, solving this use case and more.

Author: ifreund

lib/std/build.zig

@@ -1212,6 +1212,7 @@ pub const LibExeObjStep = struct {
     is_linking_libc: bool = false,
     vcpkg_bin_path: ?[]const u8 = null,
 
+    install_rel_path_override: ?[]const u8,
     installed_path: ?[]const u8,
     install_step: ?*InstallArtifactStep,
 
@@ -1348,6 +1349,7 @@ pub const LibExeObjStep = struct {
             .rdynamic = false,
             .output_dir = null,
             .single_threaded = false,
+            .install_rel_path_override = null,
             .installed_path = null,
             .install_step = null,
         };
@@ -1417,6 +1419,11 @@ pub const LibExeObjStep = struct {
         self.output_dir = self.builder.dupePath(dir);
     }
 
+    // Override the install path, relative to the install prefix
+    pub fn overrideInstallPath(self: *LibExeObjStep, rel_path: []const u8) void {
+        self.install_rel_path_override = rel_path;
+    }
+
     pub fn install(self: *LibExeObjStep) void {
         self.builder.installArtifact(self);
     }
@@ -2327,6 +2334,11 @@ pub const InstallArtifactStep = struct {
         self.step.dependOn(&artifact.step);
         artifact.install_step = self;
 
+        if (artifact.install_rel_path_override) |rel_path| {
+            builder.pushInstalledFile(.Prefix, rel_path);
+            return self;
+        }
+
         builder.pushInstalledFile(self.dest_dir, artifact.out_filename);
         if (self.artifact.isDynamicLibrary()) {
             builder.pushInstalledFile(.Lib, artifact.major_only_filename);
@@ -2348,6 +2360,13 @@ pub const InstallArtifactStep = struct {
         const self = @fieldParentPtr(Self, "step", step);
         const builder = self.builder;
 
+        if (self.artifact.install_rel_path_override) |rel_path| {
+            const full_dest_path = builder.getInstallPath(.Prefix, rel_path);
+            try builder.updateFile(self.artifact.getOutputPath(), full_dest_path);
+            self.artifact.installed_path = full_dest_path;
+            return;
+        }
+
         const full_dest_path = builder.getInstallPath(self.dest_dir, self.artifact.out_filename);
         try builder.updateFile(self.artifact.getOutputPath(), full_dest_path);
         if (self.artifact.isDynamicLibrary() and self.artifact.target.wantSharedLibSymLinks()) {