[m-rev.] for review: Replace boehm_gc/extra/if_not_there.c with shell script.

[Peter Wang]

When building Mercury natively with MinGW-w64 x86_64 under MSYS2
I saw that some process would crash in the boehm_gc directory:

  ./if_not_there mach_dep.o /bin/sh [snip]/../scripts/mgnuc [snip] ./mach_dep.c
  ^^^^Starting command^^^^

  This application has requested the Runtime to terminate it in an unusual way.
  Please contact the application's support team for more information.

The same command would not crash if I ran it manually in the shell,
though it did return to the shell before completing.

Replacing if_not_there.c by a shell script avoids the problem, such that
I was able to continue and build the rest of the compiler.  We require a
working shell on all platforms that we support, unlike Boehm GC upstream,
so I'm going to leave it at that.

boehm_gc/extra/if_not_there.sh:
	Add shell script that mimics if_not_there.c.

boehm_gc/Makefile.direct:
	Use if_not_there.sh in place of the compiled version of
	if_not_there.c.
---
 boehm_gc/Makefile.direct       |  6 ++++--
 boehm_gc/extra/if_not_there.sh | 22 ++++++++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)
 create mode 100755 boehm_gc/extra/if_not_there.sh

diff --git a/boehm_gc/Makefile.direct b/boehm_gc/Makefile.direct
index 23c12bb..116bfa6 100644
--- a/boehm_gc/Makefile.direct
+++ b/boehm_gc/Makefile.direct
@@ -481,8 +481,10 @@ if_mach: $(srcdir)/extra/if_mach.c $(srcdir)/include/private/gcconfig.h
 threadlibs: $(srcdir)/extra/threadlibs.c $(srcdir)/include/private/gcconfig.h Makefile
 	$(HOSTCC) $(HOSTCFLAGS) -o threadlibs $(srcdir)/extra/threadlibs.c
 
-if_not_there: $(srcdir)/extra/if_not_there.c
-	$(HOSTCC) $(HOSTCFLAGS) -o if_not_there $(srcdir)/extra/if_not_there.c
+# Mercury-specific: use a shell script instead of if_not_there.c
+if_not_there: $(srcdir)/extra/if_not_there.sh
+	cp $< $@
+	chmod +x $@
 
 clean:
 	rm -f gc.a *.o *.exe tests/*.o gctest gctest_dyn_link test_cpp \
diff --git a/boehm_gc/extra/if_not_there.sh b/boehm_gc/extra/if_not_there.sh
new file mode 100755
index 0000000..acac5f4
--- /dev/null
+++ b/boehm_gc/extra/if_not_there.sh
@@ -0,0 +1,22 @@
+#!/bin/sh
+# Conditionally execute a command based if the file doesn't exist.
+
+if test $# -lt 2
+then
+    echo "Usage: $0 file_name command" >&2
+    exit 1
+fi
+
+file=$1
+shift
+
+# The C version tests for read access, presumably because it is limited
+# to ANSI C.
+if test -e "$file"
+then
+    exit 0
+fi
+
+echo "^^^^Starting command^^^^"
+exec "$@"
+exit 1
-- 
2.1.2