Fix build on systems where libc relies on <linux/signal.h>

Fix build on systems where libc relies on <linux/signal.h>

Commit v4.23~10 has broken the build with Android's libc because it
relies on the UAPI headers (such as <linux/signal.h>) to define various
types, rather than copying them into the libc headers (such as <signal.h>)
like most other Linux C libraries.

* linux/linux/signal.h: Rename to linux/signal.h.in.
* Makefile.am (strace_SOURCES): Remove linux/linux/signal.h.
(EXTRA_DIST): Add linux/signal.h.in.
(DISTCLEANFILES): Add linux/linux/signal.h.
* configure.ac (AC_CACHE_CHECK): Check whether <linux/signal.h>
can be used along with <signal.h>, install linux/linux/signal.h
workaround if they cannot be used together.

Reported-by: Elliott Hughes <enh@google.com>
Closes: https://github.com/strace/strace/issues/72