app/internal: use pkg-config for finding system headers and libraries

I've resisted relying on pkg-config in the hope that hard-coded include and
library paths would suffice. However, apart from having to work around some
distro-specific differences, building with hardcoded include paths fails when
building on a buildroot:

	x86_64-buildroot-linux-gnu-gcc: ERROR: unsafe header/library path used in cross-compilation: '-I/usr/include/wayland'

(see #91)

Andri mentions a workaround (prefixing paths with "="), but that doesn't seem
to work on the BSDs.

Let's see how pkg-config fares. It's an extra dependency, but it promises to keep
us isolated from the varying paths on Linux distrobutions.

Updates #91

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/xkb/xkb_unix.go

@@ -19,9 +19,9 @@ import (
 )
 
 /*
-#cgo LDFLAGS: -lxkbcommon
+#cgo linux pkg-config: xkbcommon
 #cgo freebsd openbsd CFLAGS: -I/usr/local/include
-#cgo freebsd openbsd LDFLAGS: -L/usr/local/lib
+#cgo freebsd openbsd LDFLAGS: -L/usr/local/lib -lxkbcommon
 
 #include <stdlib.h>
 #include <xkbcommon/xkbcommon.h>