jinja2cpp: fix JINJA2CPP_CXX_STANDARD mapping with compiler.cppstd when cppstd is gnu*

[SpaceIm]

Definition of JINJA2CPP_CXX_STANDARD in CMakeToolchain was broken with compiler.cppstd set to something like gnu*:

CMake Error in CMakeLists.txt:
  The CXX_STANDARD property on target "jinja2cpp" contained an invalid value:
  "gnu20".

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• I've used a recent Conan client version close to the currently deployed.
• I've tried at least one configuration locally with the conan-center hook activated.

[SpaceIm]

license stuck

[conan-center-bot]

Conan v1 pipeline ✔️

All green in build 3 (0a06b39e8f5f3cc9e90869fa900014bfeb8dade3):

• jinja2cpp/1.2.1:
  All packages built successfully! (All logs)

• jinja2cpp/1.1.0:
  All packages built successfully! (All logs)

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Conan v2 pipeline ✔️

Note: Conan v2 builds are now mandatory. Please read our discussion about it.

All green in build 3 (0a06b39e8f5f3cc9e90869fa900014bfeb8dade3):

• jinja2cpp/1.2.1:
  All packages built successfully! (All logs)

• jinja2cpp/1.1.0:
  All packages built successfully! (All logs)