dev-util/mingw64-toolchain: prevent sys/sdt.h (systemtap) detection

Unsure which conditions are needed to reproduce exactly,
(having dev-debug/systemtap is not enough) but failed for a
user with USE=systemtap enabled globally (perhaps related
to glibc[systemtap] or gcc[systemtap]).

--disable-systemtap does not do anything to help here, the
header is checked individually by libstdc++ and then fails
given it's missing for the mingw target.

Signed-off-by: Ionen Wolkens <ionen@gentoo.org>

dev-util/mingw64-toolchain/mingw64-toolchain-11.0.0_p2.ebuild

@@ -143,6 +143,9 @@ src_compile() {
 		--without-zstd
 	)
 	${multilib} || conf_gcc+=( --disable-multilib )
+	# libstdc++ may misdetect sys/sdt.h on systemtap-enabled system and fail
+	# (not passed in conf_gcc above given it is lost in sub-configure calls)
+	local -x glibcxx_cv_sys_sdt_h=no
 
 	local conf_gcc_stage1=(
 		--enable-languages=c