Fix header includes for FreeBSD compatibility

Fixes #45

CMakeLists.txt

 if(HAVE_TIME_H)
     add_definitions(-DHAVE_TIME_H)
 endif()
+check_include_file(alloca.h HAVE_ALLOCA_H)
+if(HAVE_ALLOCA_H)
+    add_definitions(-DHAVE_ALLOCA_H)
+endif()
+check_include_file(sys/types.h HAVE_SYS_TYPES_H)
+if(HAVE_SYS_TYPES_H)
+    add_definitions(-DHAVE_SYS_TYPES_H)
+endif()
 
 if(NOT DEFINED INIT_FLAVOR)
 	if(IS_DIRECTORY "${SYSD_UNIT_DIR}" AND EXISTS "/usr/lib/systemd/systemd")

srs2.c

 #undef USE_OPENSSL
 
 #include <stdarg.h>
-#include <time.h>       /* time */
-#include <sys/types.h>  /* tyepdefs */
-#include <sys/time.h>   /* timeval / timezone struct */
 #include <string.h>		/* memcpy, strcpy, memset */
+#ifdef HAVE_ALLOCA_H
 #include <alloca.h>
+#endif
 
 #ifdef USE_OPENSSL
 #include <openssl/hmac.h>

srs2.h

 #include <stdio.h>
 #include <stdlib.h>
 #include <ctype.h>
-#ifdef __APPLE__
-    #include <sys/types.h>
+#ifdef HAVE_SYS_TYPES_H
+#include <sys/types.h>
+#endif
+#ifdef HAVE_SYS_TIME_H
+#include <sys/time.h>
+#endif
+#ifdef HAVE_TIME_H
+#include <time.h>
 #endif
-
 
 #ifndef __BEGIN_DECLS
 #define __BEGIN_DECLS