build UPDATE use c99 standard instead of gnu99

GNU extensions are not mandatory for a long time now.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 12d49d7..2c96c62 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -76,7 +76,7 @@
 set(LIBYANG_DEP_SOVERSION 2.28.0)
 set(LIBYANG_DEP_SOVERSION_MAJOR 2)
 
-set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -fvisibility=hidden -std=gnu99")
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -fvisibility=hidden -std=c99")
 
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