correction to C API

Hi there, while working with exhale as a library, I noticed that the C API is only built when EXHALE_DYN_LINK is defined. This PR enables always building the C API functions, by removing that #ifdef block, and moving them into the c++ code instead of the header (to prevent duplicate symbols being created).

I also noticed those C functions were declared with a C++-ism (having default values for parameters) so I removed that, and I included stdbool.h since C requires it (most implementations let you get away without including it, but it should be included).

Finally, this adds the external API header to the INSTALL target, it should just install to a system's default include directory, like /usr/include/.