meson: Support Windows native paths (aka backslashes) on -Dprefix

Closes: #12284

.gitlab-ci.yml

@@ -542,7 +542,7 @@ gimp-snap:
   before_script:
     - if (-not $env:MSYSTEM_PREFIX) { $env:MSYSTEM_PREFIX = if ((Get-WmiObject Win32_ComputerSystem).SystemType -like 'ARM64*') { 'clangarm64' } else { 'clang64' }}
     # FIXME:'gimpenv' have buggy code about Windows paths. See: https://gitlab.gnome.org/GNOME/gimp/-/issues/12284
-    - $GIMP_PREFIX = "$PWD\_install-$env:MSYSTEM_PREFIX".Replace('\', '/')
+    - $GIMP_PREFIX = "$PWD\_install-$env:MSYSTEM_PREFIX"
   timeout: 30m
 
 .win_environ: &WIN_ENVIRON

build/windows/2_build-gimp-msys2.ps1

@@ -33,7 +33,7 @@ if ($GITLAB_CI)
 if (-not $GIMP_PREFIX)
   {
     #FIXME:'gimpenv' have buggy code about Windows paths. See: https://gitlab.gnome.org/GNOME/gimp/-/issues/12284
-    $GIMP_PREFIX = "$PWD\..\_install".Replace('\', '/')
+    $GIMP_PREFIX = "$PWD\..\_install"
   }
 Invoke-Expression ((Get-Content .gitlab-ci.yml | Select-String 'win_environ\[' -Context 0,7) -replace '> ','' -replace '- ','')
 

meson.build

@@ -128,7 +128,7 @@ fs        = import('fs')
 
 cc        = meson.get_compiler('c')
 cxx       = meson.get_compiler('cpp')
-prefix    = get_option('prefix')
+prefix    = get_option('prefix').replace('\\', '/')
 buildtype = get_option('buildtype')
 exec_ver  = '-' + gimp_app_version