Building OLA with a GCC 6 cross-toolchain fails:

```
/usr/bin/arm-linux-g++ -DHAVE_CONFIG_H -I.   -D_LARGEFILE_SOURCE
-D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -I./include -I./include
-Wall -Wformat -W -isystem
/usr/arm-buildroot-linux-gnueabihf/sysroot/usr/include -pthread
-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os
-pthread -c -o libs/acn/e131_transmit_test.o
libs/acn/e131_transmit_test.cpp
/usr/bin/arm-linux-g++ -DHAVE_CONFIG_H -I.   -D_LARGEFILE_SOURCE
-D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -I./include -I./include
-Wall -Wformat -W -isystem
/usr/arm-buildroot-linux-gnueabihf/sysroot/usr/include -pthread
-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os
-pthread -c -o libs/acn/E131TestFramework.o
libs/acn/E131TestFramework.cpp
In file included from
/opt/ext-toolchain/arm-buildroot-linux-gnueabihf/include/c++/6.1.0/ext/string_conversions.h:41:0,
                 from
/opt/ext-toolchain/arm-buildroot-linux-gnueabihf/include/c++/6.1.0/bits/basic_string.h:5402,
                 from
/opt/ext-toolchain/arm-buildroot-linux-gnueabihf/include/c++/6.1.0/string:52,
                 from ./tools/ola_trigger/config.ypp:2:
/opt/ext-toolchain/arm-buildroot-linux-gnueabihf/include/c++/6.1.0/cstdlib:75:25:
fatal error: stdlib.h: No such file or directory
 #include_next <stdlib.h>
                         ^
compilation terminated.
```

The C++ library in GCC 6 now provides its own `<stdlib.h>` header that
wraps the C library header of the same name, so in `<cstdlib>` the
header include

```
```

has become

```
```

`#include_next` is sensitive to the order of directories in the
preprocessor's search path, so if that order is changed with `-isystem`
then the compiler can't find the right header:

```
[1] /usr/arm-buildroot-linux-gnueabihf/sysroot/usr/include
[2] /opt/ext-toolchain/arm-buildroot-linux-gnueabihf/include/c++/6.1.0
[..]
End of search list.
```

`<cstdlib>` is located in [2] whereas `<stdlib.h>` (C library header) is
in [1]. In this case, the `#include_next <stdlib.h>` statement in
`<cstdlib>`, located in [2], is evaluated **after** the search path [1],
so the compiler does not find the right system header.

The problem is that the OLA build system replaces the `-I` in the CFLAGS
from libprotobuf with `-isystem` to fix some warnings treated as errors
in the libprotobuf header files.

`-isystem` should be used to suppress warnings in system headers only
and the libprotobuf header files are not system files.

The correct fix is to compile with less restrictions and remove
`-Werror` for the build.

As using `-isystem` is reordering GCCs search path and using `-isystem`
is really not necessary, remove the faulty replacement of `-I`.

Closes: #1125