silence compiler warnings for RHEL 9 Linux

We updated ${CSILENCE} Makefile variables for entries in an attempt to
minimize compiler warnings.  This was tested using
gcc version 11.4.1 20230605 (Red Hat 11.4.1-2) (GCC).

TODO: Recheck macOS clang to see if any new warnings have occur
      under macOS clang.

1992/imc/Makefile

@@ -39,7 +39,8 @@ include ../../var.mk
 # Common C compiler warnings to silence
 #
 CSILENCE= -Wno-error -Wno-implicit-function-declaration -Wno-format \
-	-Wno-deprecated-non-prototype -Wno-implicit-int
+	  -Wno-deprecated-non-prototype -Wno-implicit-int -Wno-unused-value \
+	  -Wno-builtin-declaration-mismatch -Wno-return-type
 
 # Common C compiler warning flags
 #
@@ -85,10 +86,11 @@ CC= cc
 ifeq "$(findstring $(CLANG),${CC})" "$(CLANG)"
 #
 CSILENCE+= -Wno-error -Wno-implicit-function-declaration -Wno-format \
-	-Wno-missing-variable-declarations -Wno-poison-system-directorie \
-	-Wno-poison-system-directories -Wno-unknown-warning-option \
-	-Wno-float-conversion -Wno-shorten-64-to-32 -Wno-sign-conversion \
-	-Wno-disabled-macro-expansion
+	   -Wno-missing-variable-declarations -Wno-poison-system-directorie \
+	   -Wno-poison-system-directories -Wno-unknown-warning-option \
+	   -Wno-float-conversion -Wno-shorten-64-to-32 -Wno-sign-conversion \
+	   -Wno-disabled-macro-expansion -Wno-implicit-int-float-conversion \
+	   -Wno-unsafe-buffer-usage
 #
 CWARN+= -Weverything
 #