build(util): don't use CMake 3.19 features

Fix: #684

Author: gjasny

util/CMakeLists.txt

@@ -1,6 +1,4 @@
-add_library(util INTERFACE
-  include/prometheus/detail/base64.h
-)
+add_library(util INTERFACE)
 
 add_library(${PROJECT_NAME}::util ALIAS util)
 
@@ -14,11 +12,6 @@ target_include_directories(util
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
 )
 
-set_target_properties(util
-  PROPERTIES
-    OUTPUT_NAME ${PROJECT_NAME}-util
-)
-
 install(
   TARGETS util
   EXPORT ${PROJECT_NAME}-targets