Merge bitcoin-core/secp256k1#1113: build: Add CMake-based build system

e1eb33724c ci: Add "x86_64: Windows (VS 2022)" task (Hennadii Stepanov)
10602b0030 cmake: Export config files (Hennadii Stepanov)
5468d70964 build: Add CMake-based build system (Hennadii Stepanov)

Pull request description:

  This PR adds a [CMake](https://cmake.org/)-based build system.

  Added build instructions and examples to the [`README.md`](https://github.com/hebasto/secp256k1/blob/220628-cmake/README.md#building-with-cmake-experimental) 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

  ---

  Related PRs:
  - #315
  - #549
  - #761

  ---

  **Implementation notes**

  Minimum required CMake version is 3.1. This was required to provide [`C_STANDARD`](https://cmake.org/cmake/help/latest/prop_tgt/C_STANDARD.html) 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://gitlab.kitware.com/cmake/community/-/wikis/CMake-Versions-on-Linux-Distros)
  - https://repology.org/project/cmake/versions

  ---

  # 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 https://github.com/bitcoin-core/secp256k1/pull/1113#discussion_r916979117 |
  | `--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`:
  ![image](https://user-images.githubusercontent.com/32963518/214821305-fc3ffe82-4d05-4dd7-b2c2-7ca2d5d12e86.png)

  ## 2. `make` targets

  | Autotools | CMake |
  |---|---|
  | `make` | `make` |
  | `make check` | `make check` |
  | `make install` | `make install` * |

  * Installation of `lib/pkgconfig/libsecp256k1.pc` not implemented.

ACKs for top commit:
  theuni:
    ACK e1eb33724c.
  sipa:
    ACK e1eb33724c
  real-or-random:
    ACK e1eb33724c

Tree-SHA512: ebe2772eeb1a430a0a7ae767fb1a9a82d52d5e9bf2306956cd08f7b442c862be2539774dd10d5555817353d37d1c6add78b8fe5a85bb71239304fb42c98ff337

examples/CMakeLists.txt

@@ -0,0 +1,34 @@
+add_library(example INTERFACE)
+target_include_directories(example INTERFACE
+  ${PROJECT_SOURCE_DIR}/include
+)
+target_compile_options(example INTERFACE
+  $<$<C_COMPILER_ID:MSVC>:/wd4005>
+)
+target_link_libraries(example INTERFACE
+  $<$<PLATFORM_ID:Windows>:bcrypt>
+)
+if(SECP256K1_BUILD_SHARED)
+  target_link_libraries(example INTERFACE ${PROJECT_NAME})
+elseif(SECP256K1_BUILD_STATIC)
+  target_link_libraries(example INTERFACE ${PROJECT_NAME}_static)
+  if(MSVC)
+    target_link_options(example INTERFACE /IGNORE:4217)
+  endif()
+endif()
+
+add_executable(ecdsa_example ecdsa.c)
+target_link_libraries(ecdsa_example example)
+add_test(ecdsa_example ecdsa_example)
+
+if(SECP256K1_ENABLE_MODULE_ECDH)
+  add_executable(ecdh_example ecdh.c)
+  target_link_libraries(ecdh_example example)
+  add_test(ecdh_example ecdh_example)
+endif()
+
+if(SECP256K1_ENABLE_MODULE_SCHNORRSIG)
+  add_executable(schnorr_example schnorr.c)
+  target_link_libraries(schnorr_example example)
+  add_test(schnorr_example schnorr_example)
+endif()