Migrate build system to CMake

See #5895 for details.

This commit does not include CI or documentation changes.

cmake/CheckLibcFeatures.cmake

@@ -0,0 +1,40 @@
+include(CMakePushCheckState)
+include(CheckSourceCompiles)
+include(CheckCXXSymbolExists)
+
+function(check_glob)
+	check_cxx_symbol_exists(glob "glob.h" HAVE_GLOB)
+	return (PROPAGATE HAVE_BLOB)
+endfunction()
+
+function(check_pthread_create)
+	if (Threads_FOUND)
+		# On WASI, `pthread_create()` is always available, but always fails on triples without threading
+		# support. Probe for it while requesting the stub implementation to be hidden, otherwise we will
+		# end up always crashing at runtime on thread creation.
+		cmake_push_check_state(RESET)
+			set(CMAKE_REQUIRED_DEFINITIONS -D_WASI_STRICT_PTHREAD)
+			set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_THREAD_LIBS_INIT})
+			check_source_compiles(CXX [[
+				#include <pthread.h>
+				int main() {
+					pthread_create(0, 0, 0, 0);
+				}
+			]] HAVE_PTHREAD_CREATE)
+		cmake_pop_check_state()
+	endif()
+	return (PROPAGATE HAVE_PTHREAD_CREATE)
+endfunction()
+
+function(check_system)
+	check_cxx_symbol_exists(system "stdlib.h" HAVE_SYSTEM)
+endfunction()
+
+function(check_popen)
+	check_cxx_symbol_exists(popen "stdio.h" HAVE_POPEN)
+	if (NOT HAVE_POPEN)
+		unset(HAVE_POPEN CACHE)
+		# https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/popen-wpopen
+		check_cxx_symbol_exists(_popen "stdio.h" HAVE_POPEN)
+	endif()
+endfunction()