The HTSlib header files are located in src/htslib-1.7/htslib and are copied
to ${R_PACKAGE_DIR}/include/htslib when the package gets installed.
For convenience, some .h and .c files from Samtools are also included here
and end up in ${R_PACKAGE_DIR}/include when the package gets installed.
Note that these files come from the top-level folder of the Samtools 1.7
source tree.
