Improve flags handling for meson

This aligns this to what other tools are doing.

This is required to fix --sysroot flag even without cross compiling for example when using in combination with a hermetic toolchain.

foreign_cc/meson.bzl

@@ -68,6 +68,7 @@ def _create_meson_script(configureParameters):
     inputs = configureParameters.inputs
 
     tools = get_tools_info(ctx)
+    flags = get_flags_info(ctx)
     script = pkgconfig_script(inputs.ext_build_dirs)
 
     # CFLAGS and CXXFLAGS are also set in foreign_cc/private/cmake_script.bzl, so that meson
@@ -80,20 +81,15 @@ def _create_meson_script(configureParameters):
     if " " not in tools.cxx:
         script.append("##export_var## CXX {}".format(_absolutize(ctx.workspace_name, tools.cxx)))
 
-    # set flags same as foreign_cc/private/cc_toolchain_util.bzl
-    # cannot use get_flags_info() because bazel adds additional flags that
-    # aren't compatible with compiler or linker above
-    copts = (ctx.fragments.cpp.copts + ctx.fragments.cpp.conlyopts + getattr(ctx.attr, "copts", [])) or []
-    cxxopts = (ctx.fragments.cpp.copts + ctx.fragments.cpp.cxxopts + getattr(ctx.attr, "copts", [])) or []
-
+    copts = flags.cc
+    cxxopts = flags.cxx
     if copts:
-        script.append("##export_var## CFLAGS \"{} ${{CFLAGS:-}}\"".format(" ".join(copts).replace("\"", "'")))
+        script.append("##export_var## CFLAGS \"{} ${{CFLAGS:-}}\"".format(_join_flags_list(ctx.workspace_name, copts).replace("\"", "'")))
     if cxxopts:
-        script.append("##export_var## CXXFLAGS \"{} ${{CXXFLAGS:-}}\"".format(" ".join(cxxopts).replace("\"", "'")))
+        script.append("##export_var## CXXFLAGS \"{} ${{CXXFLAGS:-}}\"".format(_join_flags_list(ctx.workspace_name, cxxopts).replace("\"", "'")))
 
-    flags = get_flags_info(ctx)
     if flags.cxx_linker_executable:
-        script.append("##export_var## LDFLAGS \"{} ${{LDFLAGS:-}}\"".format(" ".join(flags.cxx_linker_executable).replace("\"", "'")))
+        script.append("##export_var## LDFLAGS \"{} ${{LDFLAGS:-}}\"".format(_join_flags_list(ctx.workspace_name, flags.cxx_linker_executable).replace("\"", "'")))
 
     script.append("##export_var## CMAKE {}".format(attrs.cmake_path))
     script.append("##export_var## NINJA {}".format(attrs.ninja_path))
@@ -243,3 +239,6 @@ def _absolutize(workspace_name, text, force = False):
         return "\"{}\"".format(text)
 
     return absolutize_path_in_str(workspace_name, "$EXT_BUILD_ROOT/", text, force)
+
+def _join_flags_list(workspace_name, flags):
+    return " ".join([_absolutize(workspace_name, flag) for flag in flags])