Allow appending CXXFLAGS from command line

This allows appending CXXFLAGS from the command line when building libutp.

`$ make` before patch:

```
g++ -c -DPOSIX -I . -I utp_config_lib -fno-exceptions -fno-rtti -Wall -g  utp.cpp
g++ -c -DPOSIX -I . -I utp_config_lib -fno-exceptions -fno-rtti -Wall -g  utp_utils.cpp
rm -f libutp.a
ar q libutp.a utp.o utp_utils.o
ar: creating libutp.a
ranlib libutp.a
```

`$ make` after patch:

```
g++ -c -DPOSIX -I . -I utp_config_lib -fno-exceptions -fno-rtti -Wall -g  utp.cpp
g++ -c -DPOSIX -I . -I utp_config_lib -fno-exceptions -fno-rtti -Wall -g  utp_utils.cpp
rm -f libutp.a
ar q libutp.a utp.o utp_utils.o
ar: creating libutp.a
ranlib libutp.a
```

`make CXXFLAGS=-fPIC` before patch:

```
g++ -c -DPOSIX -I . -I utp_config_lib -fPIC utp.cpp
g++ -c -DPOSIX -I . -I utp_config_lib -fPIC utp_utils.cpp
rm -f libutp.a
ar q libutp.a utp.o utp_utils.o
ar: creating libutp.a
ranlib libutp.a
```

`make CXXFLAGS=-fPIC` after patch:

```
g++ -c -DPOSIX -I . -I utp_config_lib -fno-exceptions -fno-rtti -Wall -g -fPIC utp.cpp
g++ -c -DPOSIX -I . -I utp_config_lib -fno-exceptions -fno-rtti -Wall -g -fPIC utp_utils.cpp
rm -f libutp.a
ar q libutp.a utp.o utp_utils.o
ar: creating libutp.a
ranlib libutp.a
```