Update makefile.linux

GCC broke compatibility by changing the default linking behavior to --as-needed. We have to switch it back or math breaks.

```
cc -g3 -Wall -Wconversion -ansi -pedantic   -c -o pptest.o pptest.c
pptest.c: In function 'LUAF_boxboolean':
pptest.c:41:22: warning: conversion to 'char' from 'int' may alter its value [-Wconversion]
cc -rdynamic -o pptest pptest.o -lm -ldl -llua
/usr/local/lib/liblua.a(lvm.o): In function `Arith':
lvm.c:(.text+0x485): undefined reference to `pow'
lvm.c:(.text+0x4b4): undefined reference to `floor'
/usr/local/lib/liblua.a(lvm.o): In function `luaV_execute':
lvm.c:(.text+0x22f1): undefined reference to `floor'
lvm.c:(.text+0x240e): undefined reference to `pow'
```

like so.