Use of -isystem and GCC 6.x

In the latest versions of GCC, the C++ library provides its own version of stdlib.h which wraps the original C library. It therefore uses #include_next, rather than #include which means it looks for the C stdlib.h file only in include paths which come after the one containing the original GCC6 header.

When FFTW is found in the system path, it currently adds -isystem /usr/include to the compiler flags which triggers

/usr/include/c++/6.3.1/cstdlib:75:25: fatal error: stdlib.h: No such file or directory
 #include_next <stdlib.h>
                         ^