build: Add CMake-based build system

Co-authored-by: Tim Ruffing <crypto@timruffing.de>

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()