Header files are specified by the HEADERS family of variables. Generally header files are not installed, so the noinst_HEADERS variable will be the most used. [7] All header files must be listed somewhere; missing ones will not appear in the distribution. Often it is clearest to list uninstalled headers with the rest of the sources for a program. the section called “Building a program”. Headers listed in a _SOURCES variable need not be listed in any _HEADERS variable.
Headers can be installed in includedir, oldincludedir, or pkgincludedir.
[7] However, for the case of a non-installed header file that is actually used by a particular program, we recommend listing it in the program's _SOURCES variable instead of in noinst_HEADERS. We believe this is more clear.