(compiler) Detect stdlib bundled with Perlang binaries

This provides some of the groundwork for this, mentioned in
#406:

> Distribute the (compiled) stdlib along with snapshot builds

The changes to the `Makefile` means that running `make install` will now
install the `stdlib` into the expected location. The next step is to get
the `stdlib` bundled with releases and release snapshots as well.

Makefile

@@ -61,7 +61,7 @@ docfx/docfx.exe:
 darkerfx-push:
 	rsync -av docs/templates/darkerfx/ ../darkerfx/darkerfx/
 
-install: auto-generated
+install: auto-generated stdlib
 	./scripts/local_install_linux.sh
 
 # Downloads and installs the latest snapshot from https://builds.perlang.org

release-notes/v0.4.0.md

@@ -16,6 +16,7 @@
 - Run more tests in compiled mode [[#427][427]]
 - Implement modulo operator for `BigInt` [[#432][432]]
 - Support `<<` and `>>` for bigint in compiled mode [[#440][440]]
+- Detect stdlib bundled with Perlang binaries [[#444][444]]
 
 ### Changed
 #### Data types
@@ -79,3 +80,4 @@
 [440]: https://github.com/perlang-org/perlang/pull/440
 [442]: https://github.com/perlang-org/perlang/pull/442
 [443]: https://github.com/perlang-org/perlang/pull/443
+[444]: https://github.com/perlang-org/perlang/pull/444

scripts/local_install_linux.sh

@@ -11,3 +11,6 @@ mkdir -p $HOME/.perlang/nightly/bin
 
 dotnet publish src/Perlang.ConsoleApp/Perlang.ConsoleApp.csproj -c Release -r linux-x64 --self-contained true /p:PublishReadyToRun=true /p:SolutionDir=$(pwd)/
 cp -r src/Perlang.ConsoleApp/bin/Release/net7.0/linux-x64/publish/* $HOME/.perlang/nightly/bin
+
+# Copy the precompiled stdlib binaries as well, so that experimental compiled mode can find them
+cp -r lib/ $HOME/.perlang/nightly/bin

src/Perlang.Interpreter/Compiler/PerlangCompiler.cs

@@ -490,28 +490,49 @@ private void RegisterGlobalClasses()
                 }
             }
 
-            // TODO: Try picking this up from the location of the `perlang` executable first, if possible. We might even
-            // TODO: be able to get rid of this environment variable completely.
-            string? perlangRoot = Environment.GetEnvironmentVariable("PERLANG_ROOT");
+            // We attempt to detect the stdlib location by looking in the path to the `perlang` executable first. If it
+            // doesn't exist there, the PERLANG_ROOT environment variable is checked next.
+            string? executablePath = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
 
-            if (perlangRoot == null)
+            string? stdlibPath;
+
+            string perlangExecutableStdlibPath = Path.Join(executablePath, "lib", "stdlib");
+
+            if (Directory.Exists(perlangExecutableStdlibPath))
             {
-                throw new PerlangCompilerException(
-                    "The PERLANG_ROOT environment variable must be set for experimental compilation to succeed. " +
-                    "It should point to a directory where the lib/stdlib directory contains a compiled version " +
-                    "of the Perlang standard library.");
+                // This seems to be a Perlang distribution, either a nightly snapshot or a release version. Use the
+                // stdlib provided with it.
+                stdlibPath = perlangExecutableStdlibPath;
+            }
+            else
+            {
+                string? perlangRoot = Environment.GetEnvironmentVariable("PERLANG_ROOT");
+
+                if (perlangRoot == null)
+                {
+                    throw new PerlangCompilerException(
+                        $"The {perlangExecutableStdlibPath} directory does not exist, and PERLANG_ROOT is not set. One " +
+                        "of these conditions must be met for experimental compilation to succeed. If setting " +
+                        "PERLANG_ROOT, it should point to a directory where the lib/stdlib directory contains a " +
+                        "compiled version of the Perlang standard library."
+                    );
+                }
+
+                stdlibPath = Path.Join(perlangRoot, "lib", "stdlib");
             }
 
             var processStartInfo = new ProcessStartInfo
             {
-                FileName = "clang++",
+                // Make this explicit, since we have only tested this with a very specific clang version. Anything else is
+                // completely untested and not expected to work at the moment.
+                FileName = "clang++-14",
 
                 ArgumentList =
                 {
                     // Required for nested namespaces
                     "--std=c++17",
 
-                    "-I", Path.Combine(perlangRoot, "lib/stdlib/include"),
+                    "-I", Path.Combine(stdlibPath, "include"),
                     "-o", targetExecutable,
 
                     // Useful while debugging
@@ -572,7 +593,7 @@ private void RegisterGlobalClasses()
                     targetCppFile,
 
                     // TODO: Support Windows static libraries as well
-                    Path.Combine(perlangRoot, "lib/stdlib/lib/libstdlib.a"),
+                    Path.Combine(stdlibPath, "lib/libstdlib.a"),
 
                     // Needed by the Perlang stdlib
                     "-lm"