* lib/Makefile.am (coverage-clean): Remove output files left
behind by gcc -fprofile-arcs -ftest-coverage by using $(RM) with a
glob pattern rather than a $(libfindtools_a_SOURCES.c=.gcno)
Makefile substitution, because the latter caused us to delete
header files entirely (their names do not end in .c). This had
meant that it was impossible to compile findutils after "make
clean". Bug report by David Gilbert.
* find/Makefile.am (coverage-clean): Likewise.
* xargs/Makefile.am (coverage-clean): Likewise.
* locate/Makefile.am (coverage-clean): Likewise.
* Makefile.am (coverage-clean): Likewise, but also do this in the
subdirectories whose Makefile.am files are generated by
gnulib-tool.