[cmake] define STRERROR_R_CHAR_P

Summary:
This var is used in sock.cpp (syserror.cpp after D16209) to pick the correct flavor of `strerror_r` but is never defined.
Define it.

This solution is based on hebasto/bitcoin#6

Test Plan:
```
cmake .. -GNinja
ninja
```

Check the var is set in build/CMakeCache.txt and build/src/config/bitcoin-config.h

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16212

src/config/CMakeLists.txt

@@ -294,5 +294,14 @@ endif()
 
 check_type_size(__int128 __INT128)
 
+check_cxx_source_compiles("
+	#include <string.h>
+	int main() {
+	  char buf[100];
+	  char* p{strerror_r(0, buf, sizeof buf)};
+	  (void)p;
+	}
+" STRERROR_R_CHAR_P)
+
 # Generate the config
 configure_file(bitcoin-config.h.cmake.in bitcoin-config.h ESCAPE_QUOTES)

src/config/bitcoin-config.h.cmake.in

@@ -105,4 +105,7 @@
 
 #cmakedefine ENABLE_TRACING 1
 
+/* Define to 1 if strerror_r returns char *. */
+#cmakedefine STRERROR_R_CHAR_P 1
+
 #endif // BITCOIN_BITCOIN_CONFIG_H