use explicit commit for abseil

meastp · meastp · commit 629858ab1fa4 · 2019-04-03T06:54:32.000+02:00
diff --git a/cmake/OpenCensusDeps.cmake b/cmake/OpenCensusDeps.cmake
@@ -23,7 +23,7 @@ fetchcontent_declare(abseil
                      GIT_REPOSITORY
                      https://github.com/abseil/abseil-cpp
                      GIT_TAG
-                     master)
+                     2c8421e1c6cef0da9e8a20b01c15256ec9ec116d)
 fetchcontent_declare(prometheus
                      GIT_REPOSITORY
                      https://github.com/jupp0r/prometheus-cpp
