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.

Author: stanhu

ext/charlock_holmes/common.h

@@ -38,4 +38,19 @@ static inline VALUE charlock_new_str2(const char *str)
 #endif
 }
 
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+extern void Init_charlock_holmes();
+extern void _init_charlock_encoding_detector();
+extern void _init_charlock_converter();
+extern void _init_charlock_transliterator();
+
+#ifdef __cplusplus
+}
+#endif
+
 #endif

ext/charlock_holmes/ext.c

@@ -1,9 +1,5 @@
 #include "common.h"
 
-extern void _init_charlock_encoding_detector();
-extern void _init_charlock_converter();
-extern void _init_charlock_transliterator();
-
 VALUE rb_mCharlockHolmes;
 
 void Init_charlock_holmes() {
@@ -12,4 +8,4 @@ void Init_charlock_holmes() {
 	_init_charlock_encoding_detector();
 	_init_charlock_converter();
 	_init_charlock_transliterator();
-}
+}