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.

3 files changed, 23 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 $@ $^
diff --git a/cmdwrap.c b/cmdwrap.c
new file mode 100644
index 00000000..51e44b07
--- /dev/null
+++ b/cmdwrap.c
@@ -0,0 +1,17 @@
+#include <process.h>
+#include <stdlib.h>
+#include <string.h>
+
+int main(int argc, char **argv)
+{
+   if (argc != 3)
+     return EXIT_FAILURE;
+
+   if (strcmp(argv[1], "-c") != 0)
+     return EXIT_FAILURE;
+
+   argv[0] = "c:/Windows/System32/cmd.exe";
+   argv[1] = "/c";
+
+   return spawnvp(_P_WAIT, argv[0], (const char * const *) argv);
+}
diff --git a/inst.nsi b/inst.nsi
index f89facd7..41a9691d 100644
--- a/inst.nsi
+++ b/inst.nsi
@@ -47,6 +47,7 @@ section "TXR"
   SetOutPath $INSTDIR\txr\bin
   File txr.exe
   File txr-win.exe
+  File sh.exe
   File c:\cygwin\bin\cygwin1.dll
   File c:\cygwin\bin\cyggcc_s-1.dll
   SetOutPath $INSTDIR\txr\doc