build: Add CMake-based build system

[hebasto]

This PR adds a CMake-based build system.

Added build instructions and examples to the README.md file.

Ways to integrate with downstream CMake-based projects:

• if secp256k1 is a subtree (including Bitcoin Core project) -- add_subdirectory(secp256k1)
• if secp256k1 has been installed -- find_package(secp256k1 0.2.1 CONFIG), see https://github.com/hebasto/secp256k1-CMake-example

Added a few toolchain files for easy cross compiling.

Discussions on IRC:

• https://gnusha.org/secp256k1/2022-06-23.log
• https://gnusha.org/secp256k1/2022-06-24.log
• https://gnusha.org/secp256k1/2022-06-27.log
• https://gnusha.org/secp256k1/2023-01-30.log

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Related PRs:

• Simple cmake build script #315
• WIP: Add 1:1 CMake File #549
• Add cmake support #761

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Implementation notes

Minimum required CMake version is 3.1. This was required to provide C_STANDARD property.

In turn, this choice of CMake version implies it is not possible to build with default CMake on Debian 8, which has CMake v3.0.2 only.

Also see:

• CMake Versions on Linux Distros
• https://repology.org/project/cmake/versions

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Autotools -- CMake Feature Parity Tables

1. Configuration options

Autotool-based build system features being listed according to the ./configure --help output.

Autotools	CMake
--prefix	-DCMAKE_INSTALL_PREFIX
--enable-shared	-DSECP256K1_BUILD_SHARED
--enable-static	-DSECP256K1_BUILD_STATIC
--enable-dev-mode hidden	N/A, see #1113 (comment)
--enable-benchmark	-DSECP256K1_BUILD_BENCHMARK
--enable-coverage	-DCMAKE_BUILD_TYPE=Coverage
--enable-tests	-DSECP256K1_BUILD_TESTS
--enable-ctime-tests	-DSECP256K1_BUILD_CTIME_TESTS
--enable-experimental	-DSECP256K1_EXPERIMENTAL
--enable-exhaustive-tests	-DSECP256K1_BUILD_EXHAUSTIVE_TESTS
--enable-examples	-DSECP256K1_BUILD_EXAMPLES
--enable-module-ecdh	-DSECP256K1_ENABLE_MODULE_ECDH
--enable-module-recovery	-DSECP256K1_ENABLE_MODULE_RECOVERY
--enable-module-extrakeys	-DSECP256K1_ENABLE_MODULE_EXTRAKEYS
--enable-module-schnorrsig	-DSECP256K1_ENABLE_MODULE_SCHNORRSIG
--enable-external-default-callbacks	-DSECP256K1_USE_EXTERNAL_DEFAULT_CALLBACKS
--with-test-override-wide-multiply hidden	-DSECP256K1_TEST_OVERRIDE_WIDE_MULTIPLY
--with-asm	-DSECP256K1_ASM
--with-ecmult-window	-DSECP256K1_ECMULT_WINDOW_SIZE
--with-ecmult-gen-precision	-DSECP256K1_ECMULT_GEN_PREC_BITS
--with-valgrind	-DSECP256K1_VALGRING

A screenshot of grouped options from cmake-gui:

2. make targets

Autotools	CMake
make	make
make check	make check
make install	make install *

• Installation of lib/pkgconfig/libsecp256k1.pc not implemented.