Update CPM.cmake (#164)

Author: TheLartians

cmake/CPM.cmake

@@ -1,21 +1,32 @@
-set(CPM_DOWNLOAD_VERSION 0.35.0)
+set(CPM_DOWNLOAD_VERSION 0.37.0)
 
 if(CPM_SOURCE_CACHE)
-  # Expand relative path. This is important if the provided path contains a tilde (~)
-  get_filename_component(CPM_SOURCE_CACHE ${CPM_SOURCE_CACHE} ABSOLUTE)
   set(CPM_DOWNLOAD_LOCATION "${CPM_SOURCE_CACHE}/cpm/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
 elseif(DEFINED ENV{CPM_SOURCE_CACHE})
   set(CPM_DOWNLOAD_LOCATION "$ENV{CPM_SOURCE_CACHE}/cpm/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
 else()
   set(CPM_DOWNLOAD_LOCATION "${CMAKE_BINARY_DIR}/cmake/CPM_${CPM_DOWNLOAD_VERSION}.cmake")
 endif()
 
-if(NOT (EXISTS ${CPM_DOWNLOAD_LOCATION}))
+# Expand relative path. This is important if the provided path contains a tilde (~)
+get_filename_component(CPM_DOWNLOAD_LOCATION ${CPM_DOWNLOAD_LOCATION} ABSOLUTE)
+
+function(download_cpm)
   message(STATUS "Downloading CPM.cmake to ${CPM_DOWNLOAD_LOCATION}")
   file(DOWNLOAD
        https://github.com/cpm-cmake/CPM.cmake/releases/download/v${CPM_DOWNLOAD_VERSION}/CPM.cmake
        ${CPM_DOWNLOAD_LOCATION}
   )
+endfunction()
+
+if(NOT (EXISTS ${CPM_DOWNLOAD_LOCATION}))
+  download_cpm()
+else()
+  # resume download if it previously failed
+  file(READ ${CPM_DOWNLOAD_LOCATION} check)
+  if("${check}" STREQUAL "")
+    download_cpm()
+  endif()
 endif()
 
 include(${CPM_DOWNLOAD_LOCATION})