* 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.
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@66457 138bc75d-0d04-0410-961f-82ee72b054a4