Fix command spawning on Cygwin-based standalone image.

In the standalone image, we need a /bin/sh interpreter
for the sh and open-command functions to work. The / path
resolves to the installation directory, thanks to some
sysrooting logic in the Cygwin DLL. Thus, we just need
a sh.exe program in the bin directory, side by side
with txr.exe. This sh.exe program can translate the
"sh -c command" invocation to "cmd.exe /c command".

* Makefile (sh.exe): New target, built from cmdwrap.c.

* cmdwrap.c: New file.

* inst.nsi: Install sh.exe.

1 files changed, 5 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index 0c443a1d..6cb88e3b 100644
--- a/Makefile
+++ b/Makefile
@@ -468,3 +468,8 @@ conftest.clean:
 	$(V)rm -f conftest$(EXE) conftest.[co] \
 	conftest2$(EXE) conftest[12].[oc] \
 	conftest.err conftest.syms
+
+# rule for building the Cygwin /bin/sh -> cmd.exe wrapper
+
+sh.exe: cmdwrap.c
+	$(CC) -O -o $@ $^