Add StrCat, StrJoin and the rest of //strings from abseil-cpp (#756)

From abseil-cpp version bf7fc9986e20f664958fc227547fd8d2fdcf863e

Author: wilhuff

Firestore/third_party/abseil-cpp/absl/memory/CMakeLists.txt

@@ -0,0 +1,52 @@
+#
+# Copyright 2017 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+list(APPEND MEMORY_PUBLIC_HEADERS
+  "memory.h"
+)
+
+
+absl_header_library(
+  TARGET
+    absl_memory
+  EXPORT_NAME
+    memory
+)
+
+#
+## TESTS
+#
+
+# test memory_test
+list(APPEND MEMORY_TEST_SRC
+  "memory_test.cc"
+  ${MEMORY_PUBLIC_HEADERS}
+)
+set(MEMORY_TEST_PUBLIC_LIBRARIES absl::base absl::memory)
+
+
+
+absl_test(
+  TARGET
+    memory_test
+  SOURCES
+    ${MEMORY_TEST_SRC}
+  PUBLIC_LIBRARIES
+    ${MEMORY_TEST_PUBLIC_LIBRARIES}
+)
+
+
+