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.