gcc-plugins: simplify GCC plugin-dev capability test

Linus pointed out a third of the time in the Kconfig parse stage comes
from the single invocation of cc1plus in scripts/gcc-plugin.sh [1],
and directly testing plugin-version.h for existence cuts down the
overhead a lot. [2]

This commit takes one step further to kill the build test entirely.

The small piece of code was probably intended to test the C++ designated
initializer, which was not supported until C++20.

In fact, with -pedantic option given, both GCC and Clang emit a warning.

$ echo 'class test { public: int test; } test = { .test = 1 };' | g++ -x c++ -pedantic - -fsyntax-only
<stdin>:1:43: warning: C++ designated initializers only available with '-std=c++2a' or '-std=gnu++2a' [-Wpedantic]
$ echo 'class test { public: int test; } test = { .test = 1 };' | clang++ -x c++ -pedantic - -fsyntax-only
<stdin>:1:43: warning: designated initializers are a C++20 extension [-Wc++20-designator]
class test { public: int test; } test = { .test = 1 };
                                          ^
1 warning generated.

Otherwise, modern C++ compilers should be able to build the code, and
hopefully skipping this test should not make any practical problem.

Checking the existence of plugin-version.h is still needed to ensure
the plugin-dev package is installed. The test code is now small enough
to be embedded in scripts/gcc-plugins/Kconfig.

[1] https://lore.kernel.org/lkml/CAHk-=wjU4DCuwQ4pXshRbwDCUQB31ScaeuDo1tjoZ0_PjhLHzQ@mail.gmail.com/
[2] https://lore.kernel.org/lkml/CAHk-=whK0aQxs6Q5ijJmYF1n2ch8cVFSUzU5yUM_HOjig=+vnw@mail.gmail.com/

Reported-by: Linus Torvalds <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: masahir0y

scripts/gcc-plugin.sh

@@ -9,7 +9,7 @@ menuconfig GCC_PLUGINS
 	bool "GCC plugins"
 	depends on HAVE_GCC_PLUGINS
 	depends on CC_IS_GCC
-	depends on $(success,$(srctree)/scripts/gcc-plugin.sh $(CC))
+	depends on $(success,test -e $(shell,$(CC) -print-file-name=plugin)/include/plugin-version.h)
 	default y
 	help
 	  GCC plugins are loadable modules that provide extra features to the