fix: quote template CMake variables

[skycaptain]

Changelog: Bugfix: Fix cppstd/cstd variable_watch when they are not defined.
Changelog: Bugfix: Fix cstd error reporting when a recipe does not support the required version.
Docs: Omit

This fix addresses an issue where "cppstd" is undefined for pure C recipes. Consequently, "set" sometimes leaves the
"conan_watched_std_variable" unset, leading to an error in the subsequent string comparison:

CMake Error at .../conan_toolchain.cmake:64 (if):
  if given arguments:

    "READ_ACCESS" "STREQUAL" "MODIFIED_ACCESS" "AND" "NOT" "11" "STREQUAL"

  Unknown arguments specified

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[AbrilRBS]

Great catch! I wish I had figured this would be a possibility when adding it!

Maybe adding a simple test for this would be great, let us know if you need help with that :)

[skycaptain]

Great catch! I wish I had figured this would be a possibility when adding it!

Maybe adding a simple test for this would be great, let us know if you need help with that :)

Done.

[AbrilRBS]

Thanks @skycaptain - The test also uncovered a bug in the cstd error reporting, so that's 2 for 1!

Either way, I'm proposing #17157 to fix the cstd error while removing some slight duplication

[AbrilRBS]

The current test error is due to a missconfiguration for functional tests on our side:

The provided compiler.cstd=11 requires at least msvc>=192 but version 191 provided pinging @memsharded who implemented the feature in the first place and might better know where to bump the necessary values :)

[memsharded]

Many thanks for these fixes @skycaptain !

I'll fix the test.