Add CMakeLists.txt wrapper

This is attempty to simplify using this repository few steps further. It
uses the CMake `FetchContent` module to download PHP sources, adds CMake
build files to them and patches PHP sources.

Usage:

```sh
cmake -B php-build
cmake --build php-build -j
```

Author: petk

.gitignore

@@ -8,3 +8,8 @@
 
 # All PHP downloaded archives and extracted directories.
 /php-*
+
+# CMake-related files.
+/CMakeCache.txt
+/CMakeFiles/
+/CMakeUserPresets.json

CMakeLists.txt

@@ -0,0 +1,117 @@
+# CMake-based PHP build system wrapper
+#
+# It downloads PHP tarball, adds CMake files from the cmake development
+# directory in this repository and applies patches to downloaded PHP source code
+# for running CMake commands. This file is not part of the CMake-based build
+# system itself but is only a wrapper to simplify using CMake files in this
+# repository and upstream PHP sources.
+
+cmake_minimum_required(VERSION 3.25...3.31)
+
+if(CMAKE_SOURCE_DIR PATH_EQUAL CMAKE_BINARY_DIR)
+  message(
+    FATAL_ERROR
+    "In-source builds are disabled. Please set build directory. For example:\n"
+    "  cmake -B build\n"
+    "  cmake --build build -j"
+  )
+endif()
+
+project(
+  PhpBuildSystem
+  VERSION 8.3
+  DESCRIPTION "CMake-based PHP build system"
+  HOMEPAGE_URL "https://github.com/petk/php-build-system"
+  LANGUAGES NONE
+)
+
+set(phpVersion "${PhpBuildSystem_VERSION}-dev")
+set(phpBranch "PHP-${PhpBuildSystem_VERSION}")
+
+include(FetchContent)
+
+cmake_path(
+  RELATIVE_PATH
+  CMAKE_CURRENT_BINARY_DIR
+  BASE_DIRECTORY ${CMAKE_SOURCE_DIR}
+  OUTPUT_VARIABLE relativeDir
+)
+
+# Create script for the FetchContent_Declare(PATCH_COMMAND) option below.
+file(
+  CONFIGURE
+  OUTPUT CMakeFiles/PhpBuildSystem/patch.cmake
+  CONTENT [=[
+    set(relativeDir "@relativeDir@/_deps/php-src")
+    set(destination "@CMAKE_CURRENT_BINARY_DIR@/_deps/php-src")
+
+    message(STATUS "Adding CMake files to ${relativeDir}")
+    file(COPY "@CMAKE_CURRENT_SOURCE_DIR@/cmake/" DESTINATION "${destination}")
+
+    find_package(Git QUIET)
+    if(NOT GIT_FOUND)
+      message(WARNING "Git not found. Patches are not be applied to ${relativeDir}.")
+      return()
+    endif()
+
+    string(REGEX MATCH [[^([0-9]+\.[0-9]+)]] _ "@PhpBuildSystem_VERSION@")
+    file(GLOB patches "@CMAKE_CURRENT_SOURCE_DIR@/patches/${CMAKE_MATCH_1}/*.patch")
+
+    if(NOT patches)
+      return()
+    endif()
+
+    message(STATUS "Applying patches to ${relativeDir}\n")
+
+    # Add .git directory to be able to apply patches.
+    execute_process(
+      COMMAND ${GIT_EXECUTABLE} init
+      WORKING_DIRECTORY ${destination}
+      RESULT_VARIABLE result
+      OUTPUT_VARIABLE output
+      ERROR_VARIABLE error
+      ERROR_STRIP_TRAILING_WHITESPACE
+      OUTPUT_QUIET
+    )
+    if(NOT result EQUAL 0)
+      message(FATAL_ERROR "Adding .git directory failed:\n${output}\n\n${error}")
+    endif()
+
+    execute_process(
+      COMMAND ${GIT_EXECUTABLE} apply --ignore-whitespace ${patches}
+      WORKING_DIRECTORY ${destination}
+      RESULT_VARIABLE result
+      OUTPUT_VARIABLE output
+      ERROR_VARIABLE error
+      ERROR_STRIP_TRAILING_WHITESPACE
+      OUTPUT_QUIET
+    )
+
+    if(NOT result EQUAL 0)
+      message(WARNING "Failed to apply patches:\n${output}\n\n${error}.")
+    endif()
+
+    # Clean temporary .git directory. Checks are done as safeguards.
+    if(
+      IS_DIRECTORY ${destination}/.git
+      AND EXISTS ${destination}/php.ini-development
+      AND EXISTS ${destination}/main/php_version.h
+      AND EXISTS ${destination}/CMakeLists.txt
+    )
+      file(REMOVE_RECURSE ${destination}/.git/)
+    endif()
+  ]=]
+  @ONLY
+)
+
+FetchContent_Declare(
+  php
+  URL https://github.com/php/php-src/archive/refs/heads/${phpBranch}.tar.gz
+  PATCH_COMMAND
+    ${CMAKE_COMMAND}
+    -P
+    ${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/PhpBuildSystem/patch.cmake
+)
+
+message(STATUS "Downloading PHP ${phpVersion}")
+FetchContent_MakeAvailable(php)