https://bugs.gentoo.org/938689

mbuffer use macro LIBC_(OPEN|READ|WRITE|FSTAT) whic are determined by
'objdump -T ... | awk ...', but in the case of clag (at least for now),
depend on the optimization levels, open|read will be optimized to other
methods which cause LIBC_(OPEN|READ) become "", then error bellow:

expected identifier or '('
   55 | int LIBC_OPEN(const char *path, int oflag, ...)

I don't know the reason to find the name of open|read|write|fstat by this
test, so use -O0 for the feature test code.

Upstream Replied:
> this concept is needed for the tapedrive emulator that replicates the
> behavior of specific devices.

diff --git a/configure.in b/configure.in
index 95d6772..081625a 100644
--- a/configure.in
+++ b/configure.in
@@ -155,6 +155,8 @@ AC_LANG(C)
 if test -z "$OBJDUMP"; then
 	AC_MSG_WARN([unable to find objdump, which is needed to run tests])
 else
+	old_CFLAGS="${CFLAGS}"
+	CFLAGS="-O0"
 	AC_MSG_CHECKING([linking open() and write() to detect libc names])
 	AC_LINK_IFELSE([
 	AC_LANG_SOURCE([[
@@ -191,6 +193,7 @@ else
 	],
 	[AC_MSG_FAILURE([failed to link open/write test])]
 	)
+	CFLAGS="${old_CFLAGS}"
 fi