Thanks for some insights.
I reverted the commit to get the compilation error-free.
Some observations with the commit added
1) /lib/error.h is not compiled from /lib/error.in.h
It is there when the commit is abset.
2) with the commit, the config.log contains
GL_GENERATE_ERROR_H_FALSE=''
GL_GENERATE_ERROR_H_TRUE='#'
and the Makefile contains
# We need the following in order to create <error.h> when the system
# doesn't have one that works.
#lib/error.h: lib/error.in.h $(top_builddir)/config.status $(CXXDEFS_H)
# $(AM_V_GEN)$(MKDIR_P) 'lib'
# $(gl_V_at)$(SED_HEADER_STDOUT) \
# -e 's|@''HAVE_ERROR''@|$(HAVE_ERROR)|g' \
# -e 's|@''HAVE_ERROR_AT_LINE''@|$(HAVE_ERROR_AT_LINE)|g' \
# -e 's|@''REPLACE_ERROR''@|$(REPLACE_ERROR)|g' \
# -e 's|@''REPLACE_ERROR_AT_LINE''@|$(REPLACE_ERROR_AT_LINE)|g' \
# -e '/definitions of _GL_FUNCDECL_RPL/r $(CXXDEFS_H)' \
# $(top_srcdir)/lib/error.in.h > $@-t
# $(AM_V_at)mv $@-t $@
lib/error.h: $(top_builddir)/config.status
rm -f $@
