t/110-compile: remove hard dependency on /usr/include/stdio.h.

This was testing source files outside the current build tree, but if
you don't have the file at all, that's not a good reason to fail the
test.

t/110-compile/hello.o.do

@@ -1,4 +1,11 @@
-redo-ifchange CC hello.c /usr/include/stdio.h
+# This test is meant to confirm some basic redo functionality
+# related to static files not in the build tree. But if your
+# system doesn't happen to have stdio.h in the usual location,
+# let's not explode just for that.
+stdio=/usr/include/stdio.h
+[ -e "$stdio" ] || stdio=
+
+redo-ifchange CC hello.c $stdio
 redo-ifcreate stdio.h
 ../sleep 3
 ./CC hello.c