Refactor build flags handling and add tool alias management

- Introduced a new method `apply_default_string` to streamline the application of default values for string fields in build flags.
- Enhanced `apply_flags_table` to utilize the new method for setting various build flags, improving code readability and maintainability.
- Added support for additional build flags: `fuse_ld`, `ranlib`, `strip`, `nm`, `objcopy`, `objdump`, and `readelf`.
- Implemented a new command `set` to manage tool aliases for compilers, linkers, and shells, allowing users to set preferred tools in the build environment.
- Improved error handling and validation for tool alias configuration, ensuring that existing non-symlink files are not replaced.
- Added tests to verify the correct behavior of tool alias management and build flag application.

contrib/depot.d/build.toml

@@ -5,10 +5,17 @@
 # apply everywhere without crowding the main /etc/depot.toml.
 
 # Toolchain selection.
+# tool_dir = "/usr/bin"
 cc = "clang"
-# cxx = "clang++"
-# ar = "ar"
-# ld = "ld.lld"
+# cxx = "$TOOL_DIR/clang++"
+# ar = "$TOOL_DIR/ar"
+# ranlib = "$TOOL_DIR/ranlib"
+# strip = "$TOOL_DIR/strip"
+# ld = "$TOOL_DIR/ld.lld"
+# nm = "$TOOL_DIR/nm"
+# objcopy = "$TOOL_DIR/objcopy"
+# objdump = "$TOOL_DIR/objdump"
+# readelf = "$TOOL_DIR/readelf"
 
 # Compiler defaults. These are merged with package-level flags unless a package
 # explicitly replaces them.
@@ -18,6 +25,8 @@ cflags += ["-pipe"]
 
 # Linker defaults.
 ldflags = ["-Wl,-O1"]
+# Select a linker through compiler drivers such as clang/gcc.
+# fuse_ld = "lld"
 
 # Link-time optimization policy.
 ltoflags = ["-flto=auto"]

contrib/depot.toml

@@ -16,15 +16,20 @@ prefix = "/usr"
 
 # Default toolchain commands. These are passed through to builders that honor
 # CC/CXX/AR and related variables.
+# tool_dir = "/usr/bin"
 cc = "clang"
 ar = "ar"
 
 # Optional examples:
-# cxx = "clang++"
-# ld = "ld.lld"
-# cpp = "cpp"
-# strip = "strip"
-# ranlib = "ranlib"
+# cxx = "$TOOL_DIR/clang++"
+# ld = "$TOOL_DIR/ld.lld"
+# cpp = "$TOOL_DIR/cpp"
+# strip = "$TOOL_DIR/strip"
+# ranlib = "$TOOL_DIR/ranlib"
+# nm = "$TOOL_DIR/nm"
+# objcopy = "$TOOL_DIR/objcopy"
+# objdump = "$TOOL_DIR/objdump"
+# readelf = "$TOOL_DIR/readelf"
 
 [build.flags]
 # Baseline C compiler flags for packages that do not supply their own.
@@ -39,6 +44,8 @@ cflags += ["-g"]
 # Optional C++ and linker defaults:
 # cxxflags = ["-O2"]
 ldflags = ["-Wl,-O1"]
+# Select a linker through compiler drivers such as clang/gcc.
+# fuse_ld = "lld"
 
 # Link-time optimization defaults. When use_lto is true, Depot will inject
 # ltoflags into the appropriate compiler and linker flag sets automatically.

contrib/user.depot.toml.example

@@ -10,8 +10,11 @@
 prefix = "/usr/local"
 
 # Personal toolchain preference.
+# tool_dir = "/usr/bin"
 cc = "clang"
-# cxx = "clang++"
+# cxx = "$TOOL_DIR/clang++"
+# ranlib = "$TOOL_DIR/ranlib"
+# strip = "$TOOL_DIR/strip"
 
 [build.flags]
 # User-local compiler preferences.
@@ -22,6 +25,7 @@ carch = "x86_64"
 # Examples:
 # cxxflags = ["-O2", "-g"]
 # ldflags = ["-Wl,-O1"]
+# fuse_ld = "lld"
 # makeflags = "-j4"
 
 # Optional install directory overrides for supported builders.