buildsys: add check for asprintf to enable alt implementation

Some platforms (like Solaris 10) lack asprintf.  The implementation made
for Windows works fine there, so use autoconf to determine whether
asprintf exists, and if not, enable compiling the replacement
implementation.

configure.ac

@@ -485,6 +485,7 @@ AC_SUBST([FNMATCH_CPPFLAGS])
 AC_CHECK_HEADERS(fnmatch.h)
 AM_CONDITIONAL([HAVE_FNMATCH], [test "x$have_fnmatch" = "xyes"])
 
+AC_CHECK_FUNCS(asprintf)
 AC_CHECK_FUNCS(strstr)
 AC_CHECK_FUNCS(strcasecmp stricmp, break)
 AC_CHECK_FUNCS(strncasecmp strnicmp, break)

main/options.c

@@ -597,7 +597,7 @@ static vString* getHome (void)
 	}
 }
 
-#if defined(_WIN32) && !(defined(__USE_MINGW_ANSI_STDIO) && defined(__MINGW64_VERSION_MAJOR))
+#if defined(_WIN32) && !(defined(__USE_MINGW_ANSI_STDIO) && defined(__MINGW64_VERSION_MAJOR)) || !defined(HAVE_ASPRINTF)
 
 /* Some versions of MinGW are missing _vscprintf's declaration, although they
  * still provide the symbol in the import library.