Boilerplate: Adapt system / popen commands for Windows

Both system() and popen() invoke the command interpreter;
that's CMD.exe on Windows (or whatever is specified in the
COMSPEC environment variable).

The Windows API supports forward slashes in paths just fine,
but CMD doesn't (well, it does in very specific cases [1]).
Forward slashes are always parsed as start of switches / options.

[1] https://devblogs.microsoft.com/oldnewthing/20180605-00/?p=98915

boilerplate/cairo-boilerplate-pdf.c

@@ -171,8 +171,13 @@ _cairo_boilerplate_pdf_surface_write_to_png (cairo_surface_t *surface,
     char    command[4096];
     int exitstatus;
 
-    sprintf (command, "./pdf2png %s %s 1",
-	     ptc->filename, filename);
+    sprintf (command,
+#ifndef _WIN32
+             "./pdf2png %s %s 1",
+#else
+             ".\\pdf2png %s %s 1",
+#endif
+             ptc->filename, filename);
 
     exitstatus = system (command);
 #if _XOPEN_SOURCE && HAVE_SIGNAL_H

boilerplate/cairo-boilerplate-svg.c

@@ -186,8 +186,13 @@ _cairo_boilerplate_svg_surface_write_to_png (cairo_surface_t *surface,
     char    command[4096];
     int exitstatus;
 
-    sprintf (command, "./svg2png %s %s",
-	     ptc->filename, filename);
+    sprintf (command,
+#ifndef _WIN32
+             "./svg2png %s %s",
+#else
+             ".\\svg2png %s %s",
+#endif
+             ptc->filename, filename);
 
     exitstatus = system (command);
 #if _XOPEN_SOURCE && HAVE_SIGNAL_H

boilerplate/cairo-boilerplate.c

@@ -906,8 +906,13 @@ cairo_boilerplate_open_any2ppm (const char   *filename,
 #endif
 
     any2ppm = getenv ("ANY2PPM");
-    if (any2ppm == NULL)
-	any2ppm = "./any2ppm";
+    if (any2ppm == NULL) {
+#ifndef _WIN32
+        any2ppm = "./any2ppm";
+#else
+        any2ppm = ".\\any2ppm";
+#endif
+    }
 
 #if HAS_DAEMON
     if (flags & CAIRO_BOILERPLATE_OPEN_NO_DAEMON)