DOC: remove references to CPU=native in the README

Certain compilers running in virtualized environments may produce code
that the same processor cannot execute with -march=native, either because
of hypervisor bugs reporting wrong CPU features, or because of compiler
bugs forgetting to check CPU features. So better stop recommending this
combination so that users don't get trapped anymore.

README

@@ -55,8 +55,9 @@ one of the following choices to the CPU variable :
   - i686 for intel PentiumPro, Pentium 2 and above, AMD Athlon
   - i586 for intel Pentium, AMD K6, VIA C3.
   - ultrasparc : Sun UltraSparc I/II/III/IV processor
-  - native : use the build machine's specific processor optimizations
-  - generic : any other processor or no specific optimization. (default)
+  - native : use the build machine's specific processor optimizations. Use with
+    extreme care, and never in virtualized environments (known to break).
+  - generic : any other processor or no CPU-specific optimization. (default)
 
 Alternatively, you may just set the CPU_CFLAGS value to the optimal GCC options
 for your platform.
@@ -134,11 +135,11 @@ And I build it this way on OpenBSD or FreeBSD :
 
 And on a classic Linux with SSL and ZLIB support (eg: Red Hat 5.x) :
 
-    $ make TARGET=linux26 CPU=native USE_PCRE=1 USE_OPENSSL=1 USE_ZLIB=1
+    $ make TARGET=linux26 USE_PCRE=1 USE_OPENSSL=1 USE_ZLIB=1
 
 And on a recent Linux >= 2.6.28 with SSL and ZLIB support :
 
-    $ make TARGET=linux2628 CPU=native USE_PCRE=1 USE_OPENSSL=1 USE_ZLIB=1
+    $ make TARGET=linux2628 USE_PCRE=1 USE_OPENSSL=1 USE_ZLIB=1
 
 In order to build a 32-bit binary on an x86_64 Linux system with SSL support
 without support for compression but when OpenSSL requires ZLIB anyway :