meson-options: Make it possible to call this script from anywhere

Resolve the directory where meson_options.txt is located. This is
helpful for running the script in out-of-tree builds.

v2: Use __file__ (Dylan) and simplify path evaluation

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
40 jobs for !1870 with meson-allow-true-false-for-autofeatures in 16 minutes and 47 seconds (queued for 4 seconds)
latest detached
Status Job ID Name Coverage
  Containers
passed #696618
debian-10

00:00:19

passed #696619
debian-9

00:00:22

passed #696620
aarch64
test-container:arm64

00:00:22

 
  Build
passed #696623
meson-arm64

00:05:55

passed #696622
meson-armhf

00:04:02

passed #696625
meson-clang

00:09:05

passed #696628
meson-clover

00:06:43

passed #696629
meson-clover-old-llvm

00:03:44

passed #696631
meson-i386

00:05:41

passed #696621
meson-main

00:05:05

passed #696624
meson-swr-glvnd

00:05:19

passed #696630
meson-vulkan

00:02:11

passed #696632
scons

00:08:55

passed #696633
scons-old-llvm

00:06:52

passed #696626
scons-swr

00:05:10

passed #696627
scons-win64

00:07:29

 
  Test
passed #696654
db410c
arm64_a306_gles2 1/4

00:04:12

passed #696655
db410c
arm64_a306_gles2 2/4

00:05:56

passed #696656
db410c
arm64_a306_gles2 3/4

00:09:59

passed #696657
db410c
arm64_a306_gles2 4/4

00:04:25

passed #696643
mesa-cheza
arm64_a630_gles2

00:04:44

passed #696644
mesa-cheza
arm64_a630_gles31 1/4

00:06:12

passed #696645
mesa-cheza
arm64_a630_gles31 2/4

00:07:19

passed #696646
mesa-cheza
arm64_a630_gles31 3/4

00:07:23

passed #696647
mesa-cheza
arm64_a630_gles31 4/4

00:07:16

passed #696648
mesa-cheza
arm64_a630_gles3 1/6

00:04:46

passed #696649
mesa-cheza
arm64_a630_gles3 2/6

00:04:26

passed #696650
mesa-cheza
arm64_a630_gles3 3/6

00:03:50

passed #696651
mesa-cheza
arm64_a630_gles3 4/6

00:04:09

passed #696652
mesa-cheza
arm64_a630_gles3 5/6

00:04:33

passed #696653
mesa-cheza
arm64_a630_gles3 6/6

00:04:01

passed #696634
test-llvmpipe-gles2 1/4

00:03:33

passed #696635
test-llvmpipe-gles2 2/4

00:03:44

passed #696636
test-llvmpipe-gles2 3/4

00:03:25

passed #696637
test-llvmpipe-gles2 4/4

00:03:37

passed #696638
test-softpipe-gles2 1/4

00:02:03

passed #696639
test-softpipe-gles2 2/4

00:01:30

passed #696640
test-softpipe-gles2 3/4

00:01:34

passed #696641
test-softpipe-gles2 4/4

00:01:29

passed #696642
test-softpipe-gles3-limited

00:03:49