Move the -Wmissing-prototypes option from library/CMakeLists.txt to the top-level CMakeLists.txt for GCC & Clang

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index 37a9724..3f59c3c 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -197,11 +197,11 @@
 endif()
 
 if(CMAKE_COMPILER_IS_GNUCC)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wmissing-declarations -Wmissing-prototypes")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wmissing-declarations")
 endif(CMAKE_COMPILER_IS_GNUCC)
 
 if(CMAKE_COMPILER_IS_CLANG)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wmissing-declarations -Wmissing-prototypes -Wdocumentation -Wno-documentation-deprecated-sync -Wunreachable-code")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wmissing-declarations -Wdocumentation -Wno-documentation-deprecated-sync -Wunreachable-code")
 endif(CMAKE_COMPILER_IS_CLANG)
 
 if(CMAKE_COMPILER_IS_MSVC)