Allow escaped curly brackets in values

Dominic Kempfrequested to merge
feature/escapable-curly-brackets into master

Preventing key-dependent resolution. The use case I am implementing this for is quite expressive:

( ${CMAKE_CXX_COMPILER_ID} STREQUAL Clang ) AND ( {foo} STREQUAL bar ) | cmake_guard

Without this commit, meta ini would try to find replace CMAKE_CXX_COMPILER_ID.

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