CheckSymbolExists 是CMake的公共模塊，位於/usr/share/cmake-3.5/Modules目錄下。該模塊定義了宏check_symbol_exists
MACRO(CHECK_SYMBOL_EXISTS SYMBOL FILES VARIABLE)
這個宏的作用就是，查找相關文件（FILES）裏面是否包含相關符號（SYMBOL） 如果存在則設置VARIABLE 爲1。
使用check_symbol_exists宏需要在CMake文件中包含CheckSymbolExists，即：
INCLUDE(CheckSymbolExists)
然後在用check_symbol_exists檢查symbol時候存在
比如
check_symbol_exists(SHM_HUGETLB sys/shm.h HAVE_DECL_SHM_HUGETLB)
上面的列子中就是在 sys/shm.h 文件裏面查找SHM_HUGETLB 符號。如果SHM_HUGETLB存在則定義HAVE_DECL_SHM_HUGETLB，且其值爲1.
關於check_symbol_exists宏的解釋
Provides a macro to check if a symbol exists as a function, variable, or macro in C.
check_symbol_exists
check_symbol_exists(<symbol> <files> <variable>)
Check that the <symbol> is available after including given header <files> and store the result in a <variable>. Specify the list of files in one argument as a semicolon-separated list. <variable> will be created as an internal cache variable.
If the header files define the symbol as a macro it is considered available and assumed to work. If the header files declare the symbol as a function or variable then the symbol must also be available for linking (so intrinsics may not be detected). If the symbol is a type, enum value, or intrinsic it will not be recognized (consider using CheckTypeSize or CheckCSourceCompiles). If the check needs to be done in C++, consider using CheckCXXSymbolExists instead.
The following variables may be set before calling this macro to modify the way the check is run:
CMAKE_REQUIRED_FLAGS
string of compile command line flags.
CMAKE_REQUIRED_DEFINITIONS
a ;-list of macros to define (-DFOO=bar).
CMAKE_REQUIRED_INCLUDES
a ;-list of header search paths to pass to the compiler.
CMAKE_REQUIRED_LINK_OPTIONS
a ;-list of options to add to the link command.
CMAKE_REQUIRED_LIBRARIES
a ;-list of libraries to add to the link command. See policy CMP0075.
CMAKE_REQUIRED_QUIET
execute quietly without messages.
For example:
include(CheckSymbolExists)
# Check for macro SEEK_SET
check_symbol_exists(SEEK_SET "stdio.h" HAVE_SEEK_SET)#檢查頭文件stdio.h中時候有SEEK_SET宏，若有則定義HAVE_SEEK_SET緩存變量，固定設置爲1
# Check for function fopen
check_symbol_exists(fopen "stdio.h" HAVE_FOPEN)#檢查頭文件stdio.h中時候有fopen函數，若有哲定義HAVE_FOPEN緩存變量，固定設置爲1