The HTSlib header files are located in src/htslib-1.18/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.common for
how this mechanism is implemented.

For convenience we've also copied some .h and .c files from the top-level
folder of the Samtools 1.18 source tree to this folder. These files will
end up in ${R_PACKAGE_DIR}/include/ at installation time.

Finally we've also included our own samtools-1.7-compat.h file where we
redefine things that used to be defined in Samtools 1.7 but are now gone
in Samtools 1.18. Bioconductor packages that make use of those things
can include this file to make their code compatible with version 1.18
of HTSlib and Samtools.

