build: enable warnings "-Wint-conversion" and "-Wold-style-definition"

See https://lwn.net/Articles/913505/ .

m4/compiler_options.m4

@@ -95,11 +95,14 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 		      -Wformat-nonliteral \
 		      -Wformat-security \
 		      -Wimplicit-function-declaration \
+		      -Wimplicit-int \
 		      -Winit-self \
+		      -Wint-conversion \
 		      -Wlogical-op \
 		      -Wmissing-declarations \
 		      -Wmissing-include-dirs \
 		      -Wmissing-prototypes \
+		      -Wold-style-definition \
 		      -Wparentheses-equality \
 		      -Wpointer-arith \
 		      -Wshadow \

meson.build

@@ -179,11 +179,14 @@ common_flags += cc.get_supported_arguments([
   '-Wformat-nonliteral',
   '-Wformat-security',
   '-Wimplicit-function-declaration',
+  '-Wimplicit-int',
   '-Winit-self',
+  '-Wint-conversion',
   '-Wlogical-op',
   '-Wmissing-declarations',
   '-Wmissing-include-dirs',
   '-Wmissing-prototypes',
+  '-Wold-style-definition',
   '-Wparentheses-equality',
   '-Wpointer-arith',
   '-Wshadow',