meson: work around yet more glslang braindeath

Simple work-around to https://github.com/KhronosGroup/glslang/issues/1959
is to simply brute force `/usr/include/glslang` into our include path,
that way the SPIRV includes will continue working as-is.

(We also add a separate line to help with use cases like e.g. VLC where
the location of the glslang headers are relative to the build prefix)

We do it hackily by directly accessing the compiler flags because these
directories may not necessarily exist, and meson doesn't like using
`include_directories` with nonexistant paths.
11 jobs for v1.29.1 in 47 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
failed #335624
debian aarch64
build-debian-aarch64

00:00:16

passed #335625
macos
build-macos

00:00:13

passed #335620
amd64 debian
build-ubuntu

00:00:25

passed #335621
amd64 debian
build-ubuntu-static

00:00:23

passed #335622
win32
build-win32

00:00:23

passed #335623
win64
build-win64

00:00:23

 
  Test
skipped #335630
gpu
test-gpu
skipped #335626
amd64 debian
test-ubuntu-asan
skipped #335627
amd64 debian
test-ubuntu-msan
skipped #335629
amd64 debian
test-ubuntu-scan
skipped #335628
amd64 debian
test-ubuntu-ubsan
 
Name Stage Failure
failed
build-debian-aarch64 Build
Version: 0.48.2
Source dir: /builds/videolan/libplacebo
Build dir: /builds/videolan/libplacebo/build
Build type: native build

meson.build:1:0: ERROR: Meson version is 0.48.2 but project requires >=0.49.

A full log can be found at /builds/videolan/libplacebo/build/meson-logs/meson-log.txt
ERROR: Job failed: exit code 1