README: Change gcc requirement to 4.8

Technically 4.7 is probably fine for all but debugging symbols, for which a simple version check and workaround could be added. However, since all of the CI tests run 4.8 there is no regression prevention and it would be more future maintenance trouble than it's worth.

README.md

@@ -71,8 +71,7 @@ To get and compile the source you must have `patch`, `wget`, `unzip`, `git`,
 `cmake` and `luarocks` installed, as well as a version of `autoconf`
 greater than 2.64. You also need `nasm` and of course a compiler like `gcc`
 or `clang`. If you want to cross-compile for other architectures, you need a proper
-cross-compile toolchain. Your GCC should be at least of version 4.7 for both native
-and cross compiling.
+cross-compile toolchain. Your GCC should be at least version 4.8.
 
 Users of Debian and Ubuntu can install the required packages using:
 ```