silence warnings under macOS for cc, clang and gcc

Used the following script to check for warnings
not silenced via ${CSILENCE}:

```sh
find ???? -mindepth 1 -maxdepth 1 -type d | while read dir; do
(cd "$dir"; echo =-=-= "$dir" =-=-= ; csilence -v 1 clobber all ; echo ; csilence -v 1 clobber alt)
done 2>&1 | less
```

1990/theorem/Makefile

@@ -41,7 +41,8 @@ include ../../var.mk
 CSILENCE= -Wno-deprecated-declarations -Wno-empty-body -Wno-error -Wno-unused-value \
 	-Wno-implicit-function-declaration -Wno-incompatible-pointer-types \
 	-Wno-macro-redefined -Wno-parentheses -Wno-return-type -Wno-unused-parameter \
-	-Wno-empty-translation-unit
+	-Wno-empty-translation-unit -Wno-deprecated-non-prototype -Wno-int-conversion \
+	-Wno-strict-prototypes
 
 # Common C compiler warning flags
 #
@@ -89,7 +90,7 @@ ifeq ($(CC),clang)
 CSILENCE+= -Wno-comma -Wno-double-promotion -Wno-float-conversion \
 	-Wno-missing-prototypes -Wno-missing-variable-declarations \
 	-Wno-poison-system-directories -Wno-shadow -Wno-strict-prototypes \
-	-Wno-unreachable-code -Wno-unused-macros
+	-Wno-unreachable-code -Wno-unused-macros -Wno-implicit-float-conversion
 #
 CWARN+= -Weverything
 #