Add compat/libbacktrace/

Merge the `windows` branch of https://github.com/dscho/libbacktrace
(itself a fork of https://github.com/ianlancetaylor/libbacktrace) into
compat/libbacktrace/, for use primarily by compat/mingw.c to generate
(you guessed it) stack traces.

This trick was performed by:

	git fetch https://github.com/dscho/libbacktrace windows
	git read-tree --prefix=compat/libbacktrace/ -u FETCH_HEAD
	git reset $(git commit-tree -m "Add libbacktrace" \
		-p HEAD -p FETCH_HEAD $(git write-tree))

and then amending the commit to edit the commit message ;-)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

compat/libbacktrace/.gitignore

@@ -0,0 +1,5 @@
+*~
+*.o
+*.lo
+*.a
+*.la

compat/libbacktrace/Isaac.Newton-Opticks.txt

@@ -0,0 +1,29 @@
+# Copyright (C) 2012-2016 Free Software Foundation, Inc.
+
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+
+#     (1) Redistributions of source code must retain the above copyright
+#     notice, this list of conditions and the following disclaimer. 
+
+#     (2) Redistributions in binary form must reproduce the above copyright
+#     notice, this list of conditions and the following disclaimer in
+#     the documentation and/or other materials provided with the
+#     distribution.  
+    
+#     (3) The name of the author may not be used to
+#     endorse or promote products derived from this software without
+#     specific prior written permission.
+
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+# WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+# DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
+# INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+# (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+# SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+# STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
+# IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+# POSSIBILITY OF SUCH DAMAGE.