The HTSlib header files are located in src/htslib-1.7/htslib/ and will be copied
to ${R_PACKAGE_DIR}/include/htslib/ at installation time. See the mk-include-dir
and populate-include-dir targets in src/Makevars for how this mechanism is
implemented.

For convenience, some .h and .c files from Samtools are also included here
and will end up in ${R_PACKAGE_DIR}/include/ at installation time. Note that
these files come from the top-level folder of the Samtools 1.7 source tree.

