Pass DOWNLOAD_ONLY flag when CPM_pkg_SOURCE is set. (#308)

https://github.com/cpm-cmake/CPM.cmake/issues/307

README.md

@@ -274,15 +274,10 @@ See the [wiki](https://github.com/cpm-cmake/CPM.cmake/wiki/More-Snippets) for mo
 CPMAddPackage("gh:catchorg/Catch2@2.5.0")
 ```
 
-### [Boost ](https://github.com/boostorg/boost)
+### [Range-v3](https://github.com/ericniebler/range-v3)
 
-```CMake
+```Cmake
-CPMAddPackage(
+CPMAddPackage("gh:ericniebler/range-v3#0.11.0")
-  NAME Boost
-  VERSION 1.77.0
-  GITHUB_REPOSITORY "boostorg/boost"
-  GIT_TAG "boost-1.77.0"
-)
 ```
 
 ### [Yaml-cpp](https://github.com/jbeder/yaml-cpp)
@@ -292,12 +287,6 @@ CPMAddPackage(
 CPMAddPackage("gh:jbeder/yaml-cpp#yaml-cpp-0.6.3@0.6.3")
 ```
 
-### [Range-v3](https://github.com/ericniebler/range-v3)
-
-```Cmake
-CPMAddPackage("gh:ericniebler/range-v3#0.11.0")
-```
-
 ### [nlohmann/json](https://github.com/nlohmann/json)
 
 ```cmake
@@ -309,6 +298,19 @@ CPMAddPackage(
 )
 ```
 
+### [Boost ](https://github.com/boostorg/boost)
+
+```CMake
+# boost is a huge project and will take a while to download
+# using `CPM_SOURCE_CACHE` is strongly recommended
+CPMAddPackage(
+  NAME Boost
+  VERSION 1.77.0
+  GITHUB_REPOSITORY "boostorg/boost"
+  GIT_TAG "boost-1.77.0"
+)
+```
+
 ### [cxxopts](https://github.com/jarro2783/cxxopts)
 
 ```cmake

cmake/CPM.cmake

@@ -548,6 +548,7 @@ function(CPMAddPackage)
       EXCLUDE_FROM_ALL "${CPM_ARGS_EXCLUDE_FROM_ALL}"
       OPTIONS "${CPM_ARGS_OPTIONS}"
       SOURCE_SUBDIR "${CPM_ARGS_SOURCE_SUBDIR}"
+      DOWNLOAD_ONLY "${DOWNLOAD_ONLY}"
       FORCE True
     )
     cpm_export_variables(${CPM_ARGS_NAME})