build: don't download and extract tarball if source is already present

When building under Homebrew, we want to let Homebrew manage
downloading and extracting the tarballs. See PR #1411.

Also make sure to skip only if directory is not empty. Fix #1433.

third-party/cmake/DownloadAndExtractFile.cmake

@@ -18,6 +18,18 @@ if(NOT DEFINED TARGET)
   message(FATAL_ERROR "TARGET must be defined.")
 endif()
 
+set(SRC_DIR ${PREFIX}/src/${TARGET})
+
+# Check whether the source has been downloaded. If true, skip it.
+# Useful for external downloads like homebrew.
+if(EXISTS "${SRC_DIR}" AND IS_DIRECTORY "${SRC_DIR}")
+  file(GLOB EXISTED_FILES "${SRC_DIR}/*")
+  if(EXISTED_FILES)
+    message(STATUS "${SRC_DIR} is found and not empty, skipping download and extraction. ")
+    return()
+  endif()
+endif()
+
 # Taken from ExternalProject_Add.  Let's hope we can drop this one day when
 # ExternalProject_Add allows you to disable SHOW_PROGRESS on the file download.
 if(TIMEOUT)
@@ -71,8 +83,6 @@ endif()
 
 message(STATUS "downloading... done")
 
-set(SRC_DIR ${PREFIX}/src/${TARGET})
-
 # Slurped from a generated extract-TARGET.cmake file.
 message(STATUS "extracting...
      src='${file}'