控制流程

## 基本语法
##### 条件语句(if/else)
```cmake
if(condition)
    # 条件为真时执行
endif()

if(condition)
    # 条件为真时执行
else()
    # 条件为假时执行
endif()

if(condition1)
    # condition1 为真
elseif(condition2)
    # condition2 为真
else()
    # 都为假
endif()
```
##### 检查变量
```cmake
# 检查变量是否定义
if(DEFINED MY_VAR)
    message("MY_VAR is defined: ${MY_VAR}")
endif()

# 检查变量是否为空
set(EMPTY_VAR "")
if(NOT EMPTY_VAR)
    message("EMPTY_VAR is empty or undefined")
endif()
```
##### 字符串比较
```cmake
set(BUILD_TYPE "Debug")

# ❌ 错误：使用 EQUAL 进行字符串比较
if(BUILD_TYPE EQUAL "Debug")  # 会尝试数值比较
    message("Wrong!")
endif()

# ✅ 正确：使用 STREQUAL
if(BUILD_TYPE STREQUAL "Debug")
    message("Correct!")
endif()
```

##### 数值比较
```cmake
set(VERSION 15)

# 数值比较运算符
if(VERSION EQUAL 15)
    message("Version is 15")
endif()

if(VERSION GREATER 10)
    message("Version is greater than 10")
endif()

if(VERSION LESS 20)
    message("Version is less than 20")
endif()

if(VERSION GREATER_EQUAL 15)
    message("Version is 15 or greater")
endif()

if(VERSION LESS_EQUAL 15)
    message("Version is 15 or less")
endif()
```

##### 逻辑运算

```cmake
set(ENABLE_A ON)
set(ENABLE_B OFF)

# AND 运算
if(ENABLE_A AND ENABLE_B)
    message("Both enabled")
endif()

# OR 运算
if(ENABLE_A OR ENABLE_B)
    message("At least one enabled")
endif()

# 复杂条件
if((ENABLE_A OR ENABLE_B) AND NOT ENABLE_C)
    message("Complex condition met")
endif()
```
##### 文件和目录检查
```cmake
# 检查文件是否存在
if(EXISTS "${CMAKE_SOURCE_DIR}/config.txt")
    message("Config file found")
endif()

# 检查是否为目录
if(IS_DIRECTORY "${CMAKE_SOURCE_DIR}/src")
    message("src directory exists")
endif()

# 检查是否为符号链接
if(IS_SYMLINK "${CMAKE_SOURCE_DIR}/link")
    message("It's a symbolic link")
endif()

# 检查文件是否比另一个文件新
if("${CMAKE_SOURCE_DIR}/file1.txt" IS_NEWER_THAN "${CMAKE_SOURCE_DIR}/file2.txt")
    message("file1.txt is newer")
endif()
```

##### 平台检查
```cmake
# 检查操作系统
if(WIN32)
    message("Building on Windows")
elseif(UNIX)
    if(APPLE)
        message("Building on macOS")
    else()
        message("Building on Linux/Unix")
    endif()
endif()

# 检查编译器
if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
    message("Using GCC")
elseif(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
    message("Using Clang")
elseif(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
    message("Using MSVC")
endif()
```

## 循环
##### for...each...
```cmake
# 从 0 到 4
foreach(i RANGE 4)
    message("Number: ${i}")
endforeach()
# 输出: 0, 1, 2, 3, 4

# 从 1 到 5
foreach(i RANGE 1 5)
    message("Number: ${i}")
endforeach()
# 输出: 1, 2, 3, 4, 5

# 从 0 到 10，步长为 2
foreach(i RANGE 0 10 2)
    message("Number: ${i}")
endforeach()
# 输出: 0, 2, 4, 6, 8, 10
```
###### 遍历多个列表（ZIP）
```cmake
set(NAMES Alice Bob Charlie)
set(AGES 25 30 35)

foreach(name age IN ZIP_LISTS NAMES AGES)
    message("${name} is ${age} years old")
endforeach()

# 输出:
# Alice is 25 years old
# Bob is 30 years old
# Charlie is 35 years old
```
```cmake
# 遍历目录中的文件
file(GLOB SOURCE_FILES "src/*.cpp")

foreach(source_file ${SOURCE_FILES})
    message("Found source: ${source_file}")
endforeach()
```
##### while
```cmake
set(counter 0)

while(counter LESS 5)
    message("Counter: ${counter}")
    math(EXPR counter "${counter} + 1")
endwhile()

# 输出: 0, 1, 2, 3, 4
```
###### 实用示例：查找文件
```cmake
set(dir "${CMAKE_SOURCE_DIR}")
set(found FALSE)

while(NOT found AND NOT dir STREQUAL "/")
    if(EXISTS "${dir}/config.cmake")
        set(found TRUE)
        message("Found config at: ${dir}")
    else()
        get_filename_component(dir "${dir}" DIRECTORY)
    endif()
endwhile()
```
##### break() continue()