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.
To further simplify the guard logic, define HAVE_ASPRINTF for MinGW when
appropriate in e_msoft.h.

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/e_msoft.h

@@ -66,6 +66,10 @@ typedef enum { false, true } bool;
 # define FA_DIREC _A_SUBDIR
 # define ff_name name
 
+# if defined(__USE_MINGW_ANSI_STDIO) && defined(__MINGW64_VERSION_MAJOR)
+#  define HAVE_ASPRINTF 1
+# endif
+
 #endif
 
 #endif

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(HAVE_ASPRINTF)
 
 /* Some versions of MinGW are missing _vscprintf's declaration, although they
  * still provide the symbol in the import library.