gnu99 vs c99

[rainwoodman]

On Cori, when compiling with anaconda's cross compiling toolchain, if we use -c99, strdup will cause a segfault.

The reason is likely because strdup is not part of c99, and thus the compilation will not see a prototype to the function, causing the compiler to truncate the return value of the function to 'int' which is shorter than a true pointer on x64.

We shall update Makefile.travis; as it serves as an example for compiling in such situation.