Improve relative path check for msi

For posix-ish systems, we previously checked that a path was relative
by simpy checking if it included a '/' character. This meant that you
could not, for example, do
```
$ cat foo.substitutions
file rel/to/bar.template {
}

$ msi -I /some/path foo.substitutions
```
where our template file is located at `/some/path/rel/to/bar.template`.

Note that relateive paths work differently on Windows, so we carve out
an exception there.

modules/database/src/ioc/dbtemplate/Makefile

@@ -14,6 +14,7 @@ SRC_DIRS += $(IOCDIR)/dbtemplate
 PROD_HOST += msi
 
 msi_SRCS = msi.cpp
+msi_SYS_LIBS_WIN32 = shlwapi
 DOCS += msi.md
 
 INC += dbLoadTemplate.h

modules/database/src/ioc/dbtemplate/msi.cpp

@@ -27,6 +27,10 @@
 #include <osiFileName.h>
 #include <osiUnistd.h>
 
+#ifdef _WIN32
+#include <shlwapi.h>
+#endif
+
 #define MAX_BUFFER_SIZE 4096
 #define MAX_DEPS 1024
 
@@ -493,6 +497,14 @@ static void inputErrPrint(const inputData *const pinputData)
     EXIT;
 }
 
+static int isPathRelative(const char * const path) {
+#ifdef _WIN32
+    return path && PathIsRelativeA(path);
+#else
+    return path && path[0] != '/';
+#endif
+}
+
 static void inputOpenFile(inputData *pinputData, const char * const filename)
 {
     std::list<std::string>& pathList = pinputData->pathList;
@@ -505,7 +517,7 @@ static void inputOpenFile(inputData *pinputData, const char * const filename)
         STEP("Using stdin");
         fp = stdin;
     }
-    else if (pathList.empty() || strchr(filename, '/')){
+    else if (pathList.empty() || !isPathRelative(filename)){
         STEPS("Opening ", filename);
         fp = fopen(filename, "r");
     }