makefile: specify libs after source

It seems some recent version of gcc (4.8.2) have issues with the commandline
as stated. From the issue report:

cc -lX11   -o hhpc hhpc.o -D_XOPEN_SOURCE -D_POSIX_C_SOURCE=200809L
-pedantic -std=c99 -s -O2 -DNDEBUG

hhpc.o: In function `waitForMotion':
hhpc.c:(.text+0xd1): undefined reference to `XCreateBitmapFromData'
hhpc.c:(.text+0xf2): undefined reference to `XCreatePixmapCursor'
hhpc.c:(.text+0x100): undefined reference to `XFreePixmap'
hhpc.c:(.text+0x204): undefined reference to `XGrabPointer'
hhpc.c:(.text+0x23e): undefined reference to `XUngrabPointer'
hhpc.c:(.text+0x249): undefined reference to `XFreeCursor'
hhpc.c:(.text+0x2d9): undefined reference to `XAllowEvents'
hhpc.c:(.text+0x2e3): undefined reference to `XSync'
hhpc.c:(.text+0x340): undefined reference to `XAllowEvents'
hhpc.c:(.text+0x34a): undefined reference to `XUngrabPointer'
hhpc.c:(.text+0x361): undefined reference to `XMaskEvent'
hhpc.c:(.text+0x373): undefined reference to `XPending'
hhpc.o: In function `main':
hhpc.c:(.text.startup+0x85): undefined reference to `XOpenDisplay'
hhpc.c:(.text.startup+0xc7): undefined reference to `XCloseDisplay'
collect2: error: ld returned 1 exit status

Even though the user had the X11 libraries installed. Moving the -lX11 flag
near the end seems to hav solved the issue.

Fixes #6