Option for gmap.html location

For Fedora packaging, I moved gmap.html to /usr/share so it will run out of the box for any user. It would be nice to configure on command line (or even a config file). I will propose a patch at some point. I do NOT recommend the below patch for the repo - it is just to show what I did for Fedora.

```@@ -46,6 +46,7 @@
#include "rtl-sdr.h"
#include "anet.h"

+#define GMAP_HTML "/usr/share/dump1090/gmap.html"
#define MODES_DEFAULT_RATE 2000000
#define MODES_DEFAULT_FREQ 1090000000
#define MODES_DEFAULT_WIDTH 1000
@@ -2253,8 +2254,8 @@ int handleHTTPRequest(struct client *c)
struct stat sbuf;
int fd = -1;

- if (stat("gmap.html",&sbuf) != -1 &&
- (fd = open("gmap.html",O_RDONLY)) != -1)
+ if (stat(GMAP_HTML,&sbuf) != -1 &&
+ (fd = open(GMAP_HTML,O_RDONLY)) != -1)
{
content = malloc(sbuf.st_size);
if (read(fd,content,sbuf.st_size) == -1) {
```