find_tool() with pkgconfig seems to need some unescaping

[bugaevc]

Describe the bug

I'm trying to build glib with introspection support on Windows. After jumping through various hoops, I get this error message when building glib:

Dependency gobject-introspection-1.0 found: YES 1.83.1 (cached)
Program C:/Program\ Files/GTK\ 4\ Platform/bin/../bin/g-ir-scanner found: NO

..\girepository\introspection\meson.build:78:17: ERROR: Dependency 'gobject-introspection-1.0' tool variable 'g_ir_scanner' contains erroneous value: 'C:/Program\\ Files/GTK\\ 4\\ Platform/bin/../bin/g-ir-scanner'

The pkg-config file in question, itself produced by Meson, indeed contains:

prefix=C:/Program\ Files/GTK\ 4\ Platform
bindir=${prefix}/bin
<...>
g_ir_scanner=${bindir}/g-ir-scanner

the error seems to come from #12479, and the logic from #8272 (cc @xclaesse). Apparently the variable value should be un-escaped before you try to use it as a path?

I'm using pkgconf if it matters, but it does make sense that the pkg-config invocation reports the value without unescaping it -- otherwise you won't be able to tell the difference between a list of paths and a path containing whitespace.

To Reproduce

Build & install gobject-introspection into a prefix that contains whitespaces. Try to build glib with introspection enabled.

Expected behavior

I'd expect g-ir-scanner to get found.

system parameters

• Is this a cross build or just a plain native build (for the same computer)?
  • not a cross build
• what operating system (e.g. MacOS Catalina, Windows 10, CentOS 8.0, Ubuntu 18.04, etc.)
  • Windows 11