Use constexpr to define version numbers, not #define

[claremacrae]

This arose out of #91.

It would be better to use constexpr instead of #define to define the version number components of this project in code - for type-safety.

Not everyone who uses this project uses it with CMake, so I would stick with our current structure - but I see that constexpr is a nicer - type-safe - way of setting the various version numbers...

I'll set up a separate issue to convert the #define approach in to constexpr instead...

Originally posted by @claremacrae in #91 (comment)

[claremacrae]

Current situation:

We use this:

ApprovalTests.cpp/build/prepare_release.py

Lines 62 to 81 in d997f56

	def update_version_number_header(self):
	pushdir(self.details.approval_tests_dir)
	version_header = os.path.join("ApprovalTestsVersion.h")
	text = \
	F"""#ifndef APPROVALTESTS_CPP_APPROVALTESTSVERSION_H
	#define APPROVALTESTS_CPP_APPROVALTESTSVERSION_H
	#define APPROVALTESTS_VERSION_MAJOR {self.details.new_version_object['major']}
	#define APPROVALTESTS_VERSION_MINOR {self.details.new_version_object['minor']}
	#define APPROVALTESTS_VERSION_PATCH {self.details.new_version_object['patch']}
	#define APPROVALTESTS_VERSION_STR "{version.get_version_without_v(self.details.new_version)}"
	#define APPROVALTESTS_VERSION \\
	(APPROVALTESTS_VERSION_MAJOR * 10000 + APPROVALTESTS_VERSION_MINOR * 100 + \\
	APPROVALTESTS_VERSION_PATCH)
	#endif //APPROVALTESTS_CPP_APPROVALTESTSVERSION_H
	"""
	write_file(version_header, text)

To generate this:

ApprovalTests.cpp/ApprovalTests/ApprovalTestsVersion.h

Lines 1 to 13 in d997f56

	#ifndef APPROVALTESTS_CPP_APPROVALTESTSVERSION_H
	#define APPROVALTESTS_CPP_APPROVALTESTSVERSION_H
	#define APPROVALTESTS_VERSION_MAJOR 8
	#define APPROVALTESTS_VERSION_MINOR 4
	#define APPROVALTESTS_VERSION_PATCH 0
	#define APPROVALTESTS_VERSION_STR "8.4.0"
	#define APPROVALTESTS_VERSION \
	(APPROVALTESTS_VERSION_MAJOR * 10000 + APPROVALTESTS_VERSION_MINOR * 100 + \
	APPROVALTESTS_VERSION_PATCH)
	#endif //APPROVALTESTS_CPP_APPROVALTESTSVERSION_H

[claremacrae]

See the following template for the intention of this ticket...

https://github.com/jwillikers/cmake_configure_version_header/blob/fa644a6db6d61b7ac6dffce738bcb038fa4e6bd7/src/version.hpp.in#L1-L14

Instead of the @...@ text, our Python script would insert actual values.

We need to think about the use of a namespace, though - It would be a breaking change to add this now...

And we would use char[] instead of std::string_view

[claremacrae]

We're marking this as on_hold, as no current plans to implement it...

[claremacrae]

It doesn't have to be a breaking change, if we add this as a new mechanism, and keep the old one...