Use an LD_PRELOAD library in place of symlink to /home.

Apart from dependency on native-code DLLs, the one other quirk of
running WiX under mono is that it doesn't seem to quite get the right
answers when combining $PWD with a Unix absolute path to one of its
own supporting files - it doesn't interpret the leading slash on the
latter as meaning $PWD should be ignored, so it tries to look for
/home/my/build/dir/home/my/wix/install/dir/some.file. Previously I was
bodging around that by having my build dir contain a symlink 'home'
pointing at /home; this is marginally less intrusive.

Makefile.wixfakelibs

@@ -2,7 +2,7 @@
 
 .SUFFIXES: .c .lo .la
 
-all: libwinterop.so.la libmsi.so.la
+all: libwinterop.so.la libmsi.so.la libpreload.la
 
 %.la:
 	libtool --mode=link gcc -o $@ $^ -rpath /usr/local/lib
@@ -14,6 +14,8 @@ libwinterop.so.la: fake-winterop.lo fake-lib.lo
 
 libmsi.so.la: fake-msi.lo fake-lib.lo
 
+libpreload.la: preload.lo
+
 clean:
 	rm -rf .libs
 	rm -f *.o *.lo *.la

preload.c

@@ -0,0 +1,42 @@
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <dlfcn.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+static const char *munge(const char *n) {
+    static const char *e = NULL;
+    static size_t elen;
+    if (!e) {
+        e = getenv("WIX");
+        if (!e)
+            return n;
+        elen = strlen(e);
+    }
+
+    const char *last_slash = strrchr(n, '/');
+    if (!last_slash)
+        return n;
+    if (last_slash - n > elen && !strncmp(last_slash - elen, e, elen))
+        return last_slash - elen;
+    return n;
+}
+
+#define dofunc(rettype,sym,proto,proto2)        \
+        rettype sym proto {                     \
+            rettype (*real) proto;              \
+            real = dlsym(RTLD_NEXT, #sym);      \
+            fname = munge(fname);               \
+            return real proto2;                 \
+        }
+
+dofunc(int,open,(const char *fname,int flags,mode_t mode),(fname,flags,mode))
+dofunc(int,open64,(const char *fname,int flags,mode_t mode),(fname,flags,mode))
+dofunc(int,__open,(const char *fname,int flags,mode_t mode),(fname,flags,mode))
+dofunc(int,__open64,(const char *fname,int flags,mode_t mode),(fname,flags,mode))
+dofunc(int,stat,(const char *fname,struct stat *sbuf),(fname,sbuf))
+dofunc(int,lstat,(const char *fname,struct stat *sbuf),(fname,sbuf))
+dofunc(int,access,(const char *fname,int mode),(fname,mode))