Unbreak master builds on CentOS 5.

The problem is that the new byte-ordering macros adopted on master
don't support CentOS 5 because they assumed that any Linux system had
endian(3) support.  CentOS 6 (and presumably newer) do, but CentOS 5
doesn't.

So instead we only do glibc endian(3) support if we're on a system
with glibc 2.9 or higher (which is when this functionality was
introduced).

For any other platform that we don't detect (which now includes older
glibc such as CentOS 5), bring back our homebrewed htonll and ntohll
implementation from iperf 3.0.x.

Fixes esnet#224.

src/portable_endian.h

@@ -10,7 +10,15 @@
 
 #endif
 
-#if defined(__linux__) || defined(__CYGWIN__)
+// GLIBC / Linux with endian(3) support, which was added in glibc 2.9.
+// Intended to support CentOS 6 and newer.
+#if defined(__linux__) && \
+    ((__GLIBC__ > 3) || \
+     (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 9))
+
+#	include <endian.h>
+
+#elif defined(__CYGWIN__)
 
 #	include <endian.h>
 
@@ -117,8 +125,38 @@
 
 #else
 
+// Unsupported platforms.
+// Intended to support CentOS 5 but hopefully not too far from
+// the truth because we use the homebrew htonll, et al. implementations
+// that were originally the sole implementation of this functionality
+// in iperf 3.0.
 #	warning platform not supported
-#	include <sys/endian.h>
+#	include <endian.h>
+#if BYTE_ORDER == BIG_ENDIAN
+#define HTONLL(n) (n)
+#define NTOHLL(n) (n)
+#else
+#define HTONLL(n) ((((unsigned long long)(n) & 0xFF) << 56) | \
+                   (((unsigned long long)(n) & 0xFF00) << 40) | \
+                   (((unsigned long long)(n) & 0xFF0000) << 24) | \
+                   (((unsigned long long)(n) & 0xFF000000) << 8) | \
+                   (((unsigned long long)(n) & 0xFF00000000) >> 8) | \
+                   (((unsigned long long)(n) & 0xFF0000000000) >> 24) | \
+                   (((unsigned long long)(n) & 0xFF000000000000) >> 40) | \
+                   (((unsigned long long)(n) & 0xFF00000000000000) >> 56))
+
+#define NTOHLL(n) ((((unsigned long long)(n) & 0xFF) << 56) | \
+                   (((unsigned long long)(n) & 0xFF00) << 40) | \
+                   (((unsigned long long)(n) & 0xFF0000) << 24) | \
+                   (((unsigned long long)(n) & 0xFF000000) << 8) | \
+                   (((unsigned long long)(n) & 0xFF00000000) >> 8) | \
+                   (((unsigned long long)(n) & 0xFF0000000000) >> 24) | \
+                   (((unsigned long long)(n) & 0xFF000000000000) >> 40) | \
+                   (((unsigned long long)(n) & 0xFF00000000000000) >> 56))
+#endif
+
+#define htobe64(n) HTONLL(n)
+#define be64toh(n) NTOHLL(n)
 
 #endif