meson: fix install failure on Windows if bindir is not in PATH

We try to run fc-cache as part of the install, which runs
the fc-cache binary in the build directory. On Windows this
might fail because nothing sets up the environment in such
a way that Windows can find the libs required by fc-cache
(unless by luck we are operating in an environment where
the install bindir is already in PATH, but this is not
guaranteed).

Ideally Meson should set up the environment properly here,
but currently it doesn't do that, so until it does we need
to work around this problem somehow.

For now we simply ignore any fc-cache failures on Windows
and print a big warning message. We have to suppress the
dialog boxes that pop up when a DLL is not found in order
to make this work nicely. This isn't pretty, but seems
that least bad option for now.

See https://github.com/mesonbuild/meson/issues/8187

Fixes #260
11 jobs for meson-fix-install-fc-cache-run-on-windows in 8 minutes and 47 seconds (queued for 9 seconds)
latest
Status Job ID Name Coverage
  Build
passed #6809770
meson android arm64 fedora

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passed #6809768
gst-macos-10.15
meson macos

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passed #6809765
meson-mingw-w64-build

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manual #6809769
docker windows 1809 allowed to fail manual
meson msys2
passed #6809763
meson-shared-build

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passed #6809764
meson-static-build

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passed #6809766
docker windows 1809
meson vs2017 amd64

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passed #6809767
docker windows 1809
meson vs2017 x86

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passed #6809762
mingw-build

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passed #6809760
shared-build

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passed #6809761
static-build

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