Fix gcc6 issues - add explicit c++98 compile and link flag

[mateidavid]

As per https://gcc.gnu.org/gcc-6/changes.html , the default C++ dialect in GCC 6 is updated from GNU++98 all the way to GNU++14. This causes compile errors for C++ programs that are not C++14 (or C++11) compliant, and do not request a specific dialect. As pointed out in #115 , such issues prevent BamTools from being built with GCC 6.

This pull request explicitly adds -std=c++98 to both compile and link flags (i.e., to CXXFLAGS), which is one way of allowing BamTools 2.4.0 to be built with GCC 6.0.0.

The caveat of doing this is that the libraries produced will (if I understand correctly) use the C++98 ABI, and they might get used in a system based on the C++14 ABI. Some ABI differences are explained here: https://gcc.gnu.org/wiki/Cxx11AbiCompatibility (I couldn't find a similar document regarding C++14). A quick glance suggests libbamtools.{a,so} do not use any problematic symbols, but this is not a guarantee, and compatibility might be broken in the future.

An alternative way of making BamTools more GCC 6 friendly would be to address dialect differences, one by one.

[mateidavid]

Here is a Dockerfile based on Debian Experimental using GCC 6.0.0 demonstrating the build (thanks @mr-c):

# BamTools version 2.4.0 on Debian Experimental (gcc 6.0.0)
#
FROM debian:experimental

# install g++, make
RUN apt-get update && apt-get install -y -t experimental build-essential

# install other build tools
RUN apt-get update && apt-get install -y cmake git zlib1g-dev

# download sources
RUN mkdir /src && \
    cd /src && \
    git clone --branch add-c++98-compile-and-link-flag --depth 1 https://github.com/mateidavid/bamtools.git
#    git clone --branch v2.4.0 --depth 1 https://github.com/pezmaster31/bamtools.git

# build and install
RUN mkdir -p /src/bamtools/build && \
    cd /src/bamtools/build && \
    cmake .. && \
    make && \
    make install

[mr-c]

@mateidavid Happy to help! Yes, specifying -std=c++98 is how I was going to patch the source if it wasn't incorporated here or otherwise addressed.