Add support for Musl. (#726)

Motivation:

We would like to make this work for Musl so that we can build fully
statically linked binaries that use AsyncHTTPClient.

Modifications:

Define `_GNU_SOURCE` as a compiler argument; doing it in a source file
doesn't work properly with modular headers.

Add imports of `Musl` in appropriate places.

`Musl` doesn't have `strptime_l`, so avoid using that.

Result:

async-http-client will build for Musl.

Sources/CAsyncHTTPClient/CAsyncHTTPClient.c

@@ -15,7 +15,6 @@
 #if __APPLE__
     #include <xlocale.h>
 #elif __linux__
-    #define _GNU_SOURCE
     #include <locale.h>
 #endif
 
@@ -32,7 +31,11 @@ bool swiftahc_cshims_strptime(const char * string, const char * format, struct t
 
 bool swiftahc_cshims_strptime_l(const char * string, const char * format, struct tm * result, void * locale) {
     // The pointer cast is fine as long we make sure it really points to a locale_t.
+#ifdef __musl__
+    const char * firstNonProcessed = strptime(string, format, result);
+#else
     const char * firstNonProcessed = strptime_l(string, format, result, (locale_t)locale);
+#endif
     if (firstNonProcessed) {
         return *firstNonProcessed == 0;
     }