Updated the compilation instructions.

README.md

@@ -83,7 +83,7 @@ Then:
     git submodule init
     git submodule update
     autoreconf -i
-    CFLAGS="-O2 -Wall -march=native" ./configure <options>
+    CFLAGS="-O2 -Wall -march=native -std=gnu99" ./configure <options>
     make
 
 To compile a version that can be used accross machines, remove
@@ -91,6 +91,8 @@ To compile a version that can be used accross machines, remove
 
 To compile a debug version, replace `-O2` with `-ggdb`.
 
+Depending on your environment, replace `-std=gnu99` with `-std=c99`.
+
 Systemwide installation is optional. You may run `sgminer` from the build
 directory directly, or `make install` if you wish to install
 `sgminer` to a system location or a location you specified with `--prefix`.