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/window/egl_wayland.go

@@ -12,8 +12,9 @@ import (
 )
 
 /*
-#cgo LDFLAGS: -lwayland-egl
-#cgo CFLAGS: -I/usr/include/wayland -DMESA_EGL_NO_X11_HEADERS
+#cgo linux pkg-config: egl wayland-egl
+#cgo freebsd openbsd LDFLAGS: -lwayland-egl
+#cgo CFLAGS: -DMESA_EGL_NO_X11_HEADERS
 
 #include <EGL/egl.h>
 #include <wayland-client.h>

app/internal/window/os_wayland.go

@@ -40,7 +40,8 @@ import (
 //go:generate sed -i "1s;^;// +build linux,!android,!nowayland freebsd\\n\\n;" wayland_text_input.c
 
 /*
-#cgo LDFLAGS: -lwayland-client -lwayland-cursor
+#cgo linux pkg-config: wayland-client wayland-cursor
+#cgo freebsd openbsd LDFLAGS: -lwayland-client -lwayland-cursor
 #cgo freebsd CFLAGS: -I/usr/local/include
 #cgo freebsd LDFLAGS: -L/usr/local/lib
 

app/internal/window/os_x11.go

@@ -7,7 +7,9 @@ package window
 /*
 #cgo openbsd CFLAGS: -I/usr/X11R6/include -I/usr/local/include
 #cgo openbsd LDFLAGS: -L/usr/X11R6/lib -L/usr/local/lib
-#cgo LDFLAGS: -lX11 -lxkbcommon -lxkbcommon-x11 -lX11-xcb
+#cgo freebsd openbsd LDFLAGS: -lX11 -lxkbcommon -lxkbcommon-x11 -lX11-xcb
+#cgo linux pkg-config: x11 xkbcommon xkbcommon-x11 x11-xcb
+
 #include <stdlib.h>
 #include <locale.h>
 #include <X11/Xlib.h>