* Makefile.in (LIBGCOV): Add _gcov_merge_add.
	* gcov-io.h: Make GCOV_LINKAGE extern in libgcov and prevent resulting
	namespace clash.
	(GCOV_MERGE_FUNCTIONS): New.
	(gcov_merge_fn): Declare.
	(struct gcov_ctr_info): New field "merge".
	(__gcov_merge_add): Declare.
	* coverage.c (ctr_merge_functions): New.
	(build_ctr_info_type, build_ctr_info_value): Initialize merge field
	of gcov_ctr_info type.
	* libgcov.c (__gcov_merge_add): New.
	(gcov_exit): Call a hook to merge values of counters.

From-SVN: r66457