meson: Fix build failure when compiler is not in PATH

When cross compiling gcc/clang could not exist in PATH and Meson could
be using aarch64-linux-android-clang set in a cross file for example.
11 jobs for master in 7 minutes and 54 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #5110462
gst-macos-10.15
meson macos

00:00:51

passed #5110459
meson-mingw-w64-build

00:04:36

manual #5110463
1809 windows docker allowed to fail manual
meson msys2
passed #5110457
meson-shared-build

00:04:35

passed #5110458
meson-static-build

00:04:32

passed #5115088
1809 windows docker
meson vs2017 amd64

00:01:04

passed #5110461
1809 windows docker
meson vs2017 x86

00:01:07

passed #5110456
mingw-build

00:05:38

passed #5110454
shared-build

00:06:49

passed #5110455
static-build

00:05:12

failed #5110460
1809 windows docker
meson vs2017 amd64

00:00:13