[flang][driver] Fix Driver/isysroot.f90 test (#78478)

Check for DEFAULT_SYSROOT, because when it is set -isysroot has no
effect.

flang/test/Driver/isysroot.f90

@@ -1,6 +1,8 @@
 ! Verify that the -isysroot flag is known to the frontend and, on Darwin,
 ! is passed on to the linker.
 
+! When DEFAULT_SYSROOT is set -isysroot has no effect.
+! REQUIRES: !default_sysroot
 ! RUN: %flang -### --target=aarch64-apple-darwin -isysroot /path/to/sysroot \
 ! RUN:        %s 2>&1 | FileCheck %s --check-prefix=CHECK-DARWIN
 ! RUN: %flang -### --target=aarch64-linux-gnu -isysroot /path/to/sysroot \

flang/test/lit.cfg.py

@@ -123,6 +123,10 @@
 if config.osx_sysroot:
     isysroot_flag = ["-isysroot", config.osx_sysroot]
 
+# Check for DEFAULT_SYSROOT, because when it is set -isysroot has no effect.
+if config.default_sysroot:
+    config.available_features.add("default_sysroot")
+
 # For each occurrence of a flang tool name, replace it with the full path to
 # the build directory holding that tool.
 tools = [

flang/test/lit.site.cfg.py.in

@@ -24,6 +24,7 @@ config.linked_bye_extension = @LLVM_BYE_LINK_INTO_TOOLS@
 config.cc = "@CMAKE_C_COMPILER@"
 config.osx_sysroot = path(r"@CMAKE_OSX_SYSROOT@")
 config.targets_to_build = "@TARGETS_TO_BUILD@"
+config.default_sysroot = "@DEFAULT_SYSROOT@"
 
 import lit.llvm
 lit.llvm.initialize(lit_config, config)