Add some hardening to protect against unknown/future exploits.

doc/build-unix.txt

@@ -107,3 +107,45 @@ If you need to build Boost yourself:
 sudo su
 ./bootstrap.sh
 ./bjam install
+
+
+Security
+--------
+To help make your bitcoin installation more secure by making certain attacks impossible to
+exploit even if a vulnerability is found, you can take the following measures:
+
+* Position Independent Executable
+    Build position independent code to take advantage of Address Space Layout Randomization
+    offered by some kernels. An attacker who is able to cause execution of code at an arbitrary
+    memory location is thwarted if he doesn't know where anything useful is located.
+    The stack and heap are randomly located by default but this allows the code section to be
+    randomly located as well.
+
+    On an Amd64 processor where a library was not compiled with -fPIC, this will cause an error
+    such as: "relocation R_X86_64_32 against `......' can not be used when making a shared object;"
+
+    To build with PIE, use:
+    make -f makefile.unix ... -e PIE=1
+
+    To test that you have built PIE executable, install scanelf, part of paxutils, and use:
+    scanelf -e ./bitcoin
+
+    The output should contain:
+     TYPE
+    ET_DYN
+
+* Non-executable Stack
+    If the stack is executable then trivial stack based buffer overflow exploits are possible if
+    vulnerable buffers are found. By default, bitcoin should be built with a non-executable stack
+    but if one of the libraries it uses asks for an executable stack or someone makes a mistake
+    and uses a compiler extension which requires an executable stack, it will silently build an
+    executable without the non-executable stack protection.
+
+    To verify that the stack is non-executable after compiling use:
+    scanelf -e ./bitcoin
+
+    the output should contain:
+    STK/REL/PTL
+    RW- R-- RW-
+
+    The STK RW- means that the stack is readable and writeable but not executable.