Support EVAL CODE on CMake <3.18

Author: CraigHutchinson

cmake/CPM.cmake

@@ -505,6 +505,16 @@ function(cpm_override_fetchcontent contentName)
   set_property(GLOBAL PROPERTY ${propertyName} TRUE)
 endfunction()
 
+macro(cpm_cmake_eval)
+  set(__ARGN "${ARGN}")
+  if(COMMAND cmake_language)
+    cmake_language(EVAL CODE "${__ARGN}")
+  else()
+    file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/eval.cmake "${__ARGN}")
+    include(${CMAKE_CURRENT_BINARY_DIR}/eval.cmake)
+  endif()
+endmacro()
+
 # Download and add a package from source
 macro(CPMAddPackage)
   set(__ARGN "${ARGN}")
@@ -518,7 +528,7 @@ macro(CPMAddPackage)
     foreach(__ARG IN LISTS __ARGN)
       string(APPEND __ARGN_Quoted " [==[${__ARG}]==]")
     endforeach()
-    cmake_language(EVAL CODE "cpm_add_package_multi_arg( ${__ARGN_Quoted} )")
+    cpm_cmake_eval("cpm_add_package_multi_arg( ${__ARGN_Quoted} )")
   endif()
 endmacro()
 
@@ -944,7 +954,7 @@ function(cpm_declare_fetch PACKAGE VERSION INFO)
   foreach(__item IN LISTS ARGN)
     string(APPEND __argsQuoted " [==[${__item}]==]")
   endforeach()
-  cmake_language(EVAL CODE "FetchContent_Declare(${PACKAGE} ${__argsQuoted} )")
+  cpm_cmake_eval("FetchContent_Declare(${PACKAGE} ${__argsQuoted} )")
 endfunction()
 
 # returns properties for a package previously defined by cpm_declare_fetch