Added override flag to CXXFLAGS/CFLAGS/LDFLAGS

If you define anything in the cxxflags field in gcc.ey:
```
   cxxflags: -std=c++11 
```

...then, the following statement in Makefile will have NO EFFECT:
```
CXXFLAGS += -Wall -s -O3 -fno-rtti -fno-exceptions -flto -fdata-sections -ffunction-sections
```

You actually need to force make to accept this via the "override" flag. This patch does this for CXXFLAGS, CFLAGS, and LDFLAGS. For example:
```
override CXXFLAGS += -Wall -s -O3 -fno-rtti -fno-exceptions -flto -fdata-sections -ffunction-sections
```

This fixes extensions (most notably the Paths extension), and various other things (like re-enabling optimizations).