Avoid C++ name mangling in C code

Now that a C++ compiler is used to support later versions of ICU, we
need to surround all C code with `extern "C"` to avoid name mangling
that would cause issues with symbol resolution on macOS.

ext/charlock_holmes/converter.c

@@ -1,6 +1,8 @@
 #include "unicode/ucnv.h"
 #include "common.h"
 
+extern "C" {
+
 extern VALUE rb_mCharlockHolmes;
 static VALUE rb_cConverter;
 
@@ -55,3 +57,5 @@ void _init_charlock_converter() {
 
 	rb_define_singleton_method(rb_cConverter, "convert", rb_converter_convert, 3);
 }
+
+}

ext/charlock_holmes/encoding_detector.c

@@ -1,6 +1,8 @@
 #include "unicode/ucsdet.h"
 #include "common.h"
 
+extern "C" {
+
 extern VALUE rb_mCharlockHolmes;
 static VALUE rb_cEncodingDetector;
 
@@ -375,3 +377,5 @@ void _init_charlock_encoding_detector()
 
 	rb_define_singleton_method(rb_cEncodingDetector, "supported_encodings", rb_get_supported_encodings, 0);
 }
+
+}

ext/charlock_holmes/ext.c

@@ -1,15 +1,19 @@
 #include "common.h"
 
+extern "C" {
+
 extern void _init_charlock_encoding_detector();
 extern void _init_charlock_converter();
 extern void _init_charlock_transliterator();
 
 VALUE rb_mCharlockHolmes;
 
-void Init_charlock_holmes() {
+__attribute__((visibility ("default"))) void Init_charlock_holmes() {
 	rb_mCharlockHolmes = rb_define_module("CharlockHolmes");
 
 	_init_charlock_encoding_detector();
 	_init_charlock_converter();
 	_init_charlock_transliterator();
-}
+}
+
+}