What mean file with extension "h.in"?
These files are usually the input for autoconf which will generate final .h files.
Here's an example from PCRE:
#define PCRE_MAJOR @PCRE_MAJOR@#define PCRE_MINOR @PCRE_MINOR@#define PCRE_PRERELEASE @PCRE_PRERELEASE@#define PCRE_DATE @PCRE_DATE@Autoconf will replace all variables (@…@) with the respective values and the result will be a .h file.
Typically, a .h.in file is a header template that is filled in to become the actual header by a configure script based on the outcome of several tests for features present on the target platform.
Files ending with .in are typically template files used by a program called configure that generates a new file without the extension after substituting for variable expansions. I.e., if you're looking at a source tree that has files called, e.g. Makefile.in in the tree, then ./configure will generate a usable Makefile that can be used to "make" from source.