Use the newer one of cmake and cmake3. (pytorch#12916)

Summary:
On my devgpu, `cmake` is newer than `cmake3`. Using `cmake3` causes compilation to fail. Instead of blindly using `cmake3`, we pick the newer of the two.
Pull Request resolved: pytorch#12916

Differential Revision: D10481922

Pulled By: SsnL

fbshipit-source-id: 8340136c459e25da9f5fc4f420c7e67cadc28aff

tools/build_pytorch_libs.sh

@@ -16,10 +16,22 @@ if [ -x "$(command -v rsync)" ]; then
 fi
 
 # We test the presence of cmake3 (for platforms like CentOS and Ubuntu 14.04)
-# and use that if so.
+# and use the newer of cmake and cmake3 if so.
 CMAKE_COMMAND="cmake"
 if [[ -x "$(command -v cmake3)" ]]; then
-    CMAKE_COMMAND="cmake3"
+    if [[ -x "$(command -v cmake)" ]]; then
+        # have both cmake and cmake3, compare versions
+        CMAKE_VERSION=$(cmake --version | grep 'cmake version' | awk '{print $NF}')
+        CMAKE3_VERSION=$(cmake3 --version | grep 'cmake version' | awk '{print $NF}')
+        CMAKE3_IS_NEWER=$($PYTORCH_PYTHON -c "from distutils.version import StrictVersion; print(1 if StrictVersion(\"${CMAKE3_VERSION}\") >= StrictVersion(\"${CMAKE_VERSION}\") else 0)")
+    else
+        # don't have cmake
+        CMAKE3_IS_NEWER=1
+    fi
+    if [[ $CMAKE3_IS_NEWER == "1" ]]; then
+        CMAKE_COMMAND="cmake3"
+    fi
+    unset CMAKE_VERSION CMAKE3_VERSION CMAKE3_IS_NEWER
 fi
 
 # Options for building only a subset of the libraries