build.zig: add support for using "zig c++" as the bootstrap c++ compiler

The build was previously failing with `error: unknown command: -print-file-name=libstdc++.a`
because the command invocation was
`zig -print-file-name=libstdc++.a`
instead of
`zig c++ -print-file-name=libstdc++.a`

note: .cxx_compiler_arg1 = "" instead of undefined to avoid breaking existing setups without requiring to run cmake again.

Author: xxxbxxx

build.zig

@@ -735,7 +735,11 @@ fn addCxxKnownPath(
 ) !void {
     if (!std.process.can_spawn)
         return error.RequiredLibraryNotFound;
-    const path_padded = b.exec(&.{ ctx.cxx_compiler, b.fmt("-print-file-name={s}", .{objname}) });
+
+    const path_padded = if (ctx.cxx_compiler_arg1.len > 0)
+        b.exec(&.{ ctx.cxx_compiler, ctx.cxx_compiler_arg1, b.fmt("-print-file-name={s}", .{objname}) })
+    else
+        b.exec(&.{ ctx.cxx_compiler, b.fmt("-print-file-name={s}", .{objname}) });
     var tokenizer = mem.tokenizeAny(u8, path_padded, "\r\n");
     const path_unpadded = tokenizer.next().?;
     if (mem.eql(u8, path_unpadded, objname)) {
@@ -778,6 +782,7 @@ const CMakeConfig = struct {
     cmake_static_library_prefix: []const u8,
     cmake_static_library_suffix: []const u8,
     cxx_compiler: []const u8,
+    cxx_compiler_arg1: []const u8,
     lld_include_dir: []const u8,
     lld_libraries: []const u8,
     clang_libraries: []const u8,
@@ -844,6 +849,7 @@ fn parseConfigH(b: *std.Build, config_h_text: []const u8) ?CMakeConfig {
         .cmake_static_library_prefix = undefined,
         .cmake_static_library_suffix = undefined,
         .cxx_compiler = undefined,
+        .cxx_compiler_arg1 = "",
         .lld_include_dir = undefined,
         .lld_libraries = undefined,
         .clang_libraries = undefined,
@@ -875,6 +881,10 @@ fn parseConfigH(b: *std.Build, config_h_text: []const u8) ?CMakeConfig {
             .prefix = "#define ZIG_CXX_COMPILER ",
             .field = "cxx_compiler",
         },
+        .{
+            .prefix = "#define ZIG_CXX_COMPILER_ARG1 ",
+            .field = "cxx_compiler_arg1",
+        },
         .{
             .prefix = "#define ZIG_LLD_INCLUDE_PATH ",
             .field = "lld_include_dir",
@@ -917,7 +927,8 @@ fn parseConfigH(b: *std.Build, config_h_text: []const u8) ?CMakeConfig {
                 var it = mem.splitScalar(u8, line, '"');
                 _ = it.first(); // skip the stuff before the quote
                 const quoted = it.next().?; // the stuff inside the quote
-                @field(ctx, mapping.field) = toNativePathSep(b, quoted);
+                const trimmed = mem.trim(u8, quoted, " ");
+                @field(ctx, mapping.field) = toNativePathSep(b, trimmed);
             }
         }
         if (mem.startsWith(u8, line, "#define ZIG_LLVM_LINK_MODE ")) {

stage1/config.h.in

@@ -21,6 +21,7 @@
 #define ZIG_CMAKE_STATIC_LIBRARY_PREFIX "@CMAKE_STATIC_LIBRARY_PREFIX@"
 #define ZIG_CMAKE_STATIC_LIBRARY_SUFFIX "@CMAKE_STATIC_LIBRARY_SUFFIX@"
 #define ZIG_CXX_COMPILER "@CMAKE_CXX_COMPILER@"
+#define ZIG_CXX_COMPILER_ARG1 "@CMAKE_CXX_COMPILER_ARG1@"
 #define ZIG_DIA_GUIDS_LIB "@ZIG_DIA_GUIDS_LIB_ESCAPED@"
 #define ZIG_LLD_INCLUDE_PATH "@LLD_INCLUDE_DIRS@"
 #define ZIG_LLD_LIBRARIES "@LLD_LIBRARIES@"