README: fix and extend the supported compiler documentation.

README.md

 
 1.  a functional autotools system (`make`, `autoconf`, `automake`, `libtool`
     and `pkg-config`);
-2.  a functional c compiler that supports C11.
+2.  a functional c compiler that supports C99 (e.g. gcc or clang).
 
 Optionally, you need:
 
 | Dependency    | Install                                                          |
 |---------------|------------------------------------------------------------------|
 | autotools     | `sudo apt-get install make autoconf automake libtool pkg-config` |
-| c11 compiler  | `sudo apt-get install gcc`                                       |
+| c99 compiler  | `sudo apt-get install gcc`                                       |
 
 Optional dependencies:
 
 
 	./configure --help
 
+To use a different compiler, or compiler flags, you can specify these before
+the `configure` command. For example:
+
+	CC=clang-3.5 CFLAGS=-Wextra ./configure --prefix=/usr
+
 The `espeak-ng` and `speak-ng` programs, along with the espeak-ng voices, can
 then be built with: