std.zig: search include dir and lib dir from environment variables

lib/std/zig/system/NativePaths.zig

@@ -135,6 +135,47 @@ pub fn detect(allocator: Allocator, native_info: NativeTargetInfo) !NativePaths
         // zlib.h is in /usr/include (added above)
         // libz.so.1 is in /lib/x86_64-linux-gnu (added here)
         try self.addLibDirFmt("/lib/{s}", .{triple});
+
+        // NOTE: distro like guix doesn't use FHS, so it relies on envorinment
+        // variables (C_INCLUDE_PATH, CPLUS_INCLUDE_PATH and LIBRARY_PATH) to
+        // search for headers and libraries
+        if (process.getEnvVarOwned(allocator, "C_INCLUDE_PATH")) |c_include_path| {
+            defer allocator.free(c_include_path);
+            var it = mem.tokenize(u8, c_include_path, ":");
+            while (it.next()) |dir| {
+                try self.addIncludeDir(dir);
+            }
+        } else |err| switch (err) {
+            // NOTE: since the above code won't be executed on windows, this
+            // InvalidUtf8 error is generally unreachable
+            error.InvalidUtf8 => unreachable,
+            error.EnvironmentVariableNotFound => {},
+            error.OutOfMemory => |e| return e,
+        }
+
+        if (process.getEnvVarOwned(allocator, "CPLUS_INCLUDE_PATH")) |cplus_include_path| {
+            defer allocator.free(cplus_include_path);
+            var it = mem.tokenize(u8, cplus_include_path, ":");
+            while (it.next()) |dir| {
+                try self.addIncludeDir(dir);
+            }
+        } else |err| switch (err) {
+            error.InvalidUtf8 => unreachable,
+            error.EnvironmentVariableNotFound => {},
+            error.OutOfMemory => |e| return e,
+        }
+
+        if (process.getEnvVarOwned(allocator, "LIBRARY_PATH")) |library_path| {
+            defer allocator.free(library_path);
+            var it = mem.tokenize(u8, library_path, ":");
+            while (it.next()) |dir| {
+                try self.addLibDir(dir);
+            }
+        } else |err| switch (err) {
+            error.InvalidUtf8 => unreachable,
+            error.EnvironmentVariableNotFound => {},
+            error.OutOfMemory => |e| return e,
+        }
     }
 
     return self;