Convert cmake build to (mostly) use urls rather than git clones (firebase#852)

Exception: grpc. Due to it's use of git submodules, it's not completely trivial to convert it (though probably wouldn't be too much more work.)

This helps address our build being throttled. (Maybe. This assumes github allows more fetching of tgz's than clones.) It should also speed up our build times.

The downloaded tarballs are placed into ${PROJECT_BINARY_DIR}/downloads. This allows for eventual caching in travis.

Author: rsgowman

cmake/external/googletest.cmake

@@ -13,18 +13,14 @@
 # limitations under the License.
 
 include(ExternalProject)
-include(ExternalProjectFlags)
-
-ExternalProject_GitSource(
-  GOOGLETEST_GIT
-  GIT_REPOSITORY "https://github.com/google/googletest.git"
-  GIT_TAG "release-1.8.0"
-)
 
 ExternalProject_Add(
   googletest
 
-  ${GOOGLETEST_GIT}
+  DOWNLOAD_DIR ${PROJECT_BINARY_DIR}/downloads
+  DOWNLOAD_NAME googletest-1.8.0.tar.gz
+  URL https://github.com/google/googletest/archive/release-1.8.0.tar.gz
+  URL_HASH SHA256=58a6f4277ca2bc8565222b3bbd58a177609e9c488e8a72649359ba51450db7d8
 
   PREFIX ${PROJECT_BINARY_DIR}/external/googletest
 

cmake/external/leveldb.cmake

@@ -13,7 +13,6 @@
 # limitations under the License.
 
 include(ExternalProject)
-include(ExternalProjectFlags)
 
 if(WIN32 OR LEVELDB_ROOT)
   # If the user has supplied a LEVELDB_ROOT then just use it. Add an empty
@@ -42,16 +41,13 @@ else()
       $<$<CONFIG:Release>:${CMAKE_CXX_FLAGS_RELEASE}>"
   )
 
-  ExternalProject_GitSource(
-    LEVELDB_GIT
-    GIT_REPOSITORY "https://github.com/google/leveldb.git"
-    GIT_TAG "v1.20"
-  )
-
   ExternalProject_Add(
     leveldb
 
-    ${LEVELDB_GIT}
+    DOWNLOAD_DIR ${PROJECT_BINARY_DIR}/downloads
+    DOWNLOAD_NAME leveldb-v1.20.tar.gz
+    URL https://github.com/google/leveldb/archive/v1.20.tar.gz
+    URL_HASH SHA256=f5abe8b5b209c2f36560b75f32ce61412f39a2922f7045ae764a2c23335b6664
 
     PREFIX ${PROJECT_BINARY_DIR}/external/leveldb
 

cmake/external/nanopb.cmake

@@ -11,13 +11,6 @@
 # limitations under the License.
 
 include(ExternalProject)
-include(ExternalProjectFlags)
-
-ExternalProject_GitSource(
-  NANOPB_GIT
-  GIT_REPOSITORY "https://github.com/nanopb/nanopb.git"
-  GIT_TAG "0.3.8"
-)
 
 set(
   NANOPB_PROTOC_BIN
@@ -29,7 +22,9 @@ ExternalProject_Add(
   DEPENDS
     protobuf
 
-  ${NANOPB_GIT}
+  DOWNLOAD_DIR ${PROJECT_BINARY_DIR}/downloads
+  URL https://github.com/nanopb/nanopb/archive/nanopb-0.3.8.tar.gz
+  URL_HASH SHA256=f192c7c7cc036be36babc303b7d2315d4f62e2fe4be28c172cfed4cfa0ed5f22
 
   BUILD_IN_SOURCE ON
 

cmake/external/protobuf.cmake

@@ -13,18 +13,14 @@
 # limitations under the License.
 
 include(ExternalProject)
-include(ExternalProjectFlags)
-
-ExternalProject_GitSource(
-  PROTOBUF_GIT
-  GIT_REPOSITORY "https://github.com/google/protobuf.git"
-  GIT_TAG "v3.5.1.1"
-)
 
 ExternalProject_Add(
   protobuf
 
-  ${PROTOBUF_GIT}
+  DOWNLOAD_DIR ${PROJECT_BINARY_DIR}/downloads
+  DOWNLOAD_NAME protobuf-v3.5.11.tar.gz
+  URL https://github.com/google/protobuf/archive/v3.5.1.1.tar.gz
+  URL_HASH SHA256=56b5d9e1ab2bf4f5736c4cfba9f4981fbc6976246721e7ded5602fbaee6d6869
 
   PREFIX ${PROJECT_BINARY_DIR}/external/protobuf