gdb/Makefile.in: Add DEBUGINFOD_CFLAGS, DEBUGINFOD_LIBS variables.

gdb/Makefile.in: Add DEBUGINFOD_CFLAGS, DEBUGINFOD_LIBS variables.

Introduce Makefile variables DEBUGINFOD_CFLAGS and DEBUGINFOD_LIBS
that map to the configuration variables of the same names.

Replace @DEBUGINFOD_LIBS@ with $(DEBUGINFOD_LIBS) in the definition
of CLIBS in order to conform to the usage of other *_LIBS variables
in Makefile.in.

Add DEBUGINFOD_CFLAGS to INTERNAL_CFLAGS_BASE. This fixes an issue
where GDB would fail to find debuginfod.h if it was not installed
in a default location searched by the compiler.

gdb/ChangeLog:

	* Makefile.in (DEBUGINFOD_CFLAGS, DEBUGINFOD_LIBS): New variables.
	(INTERNAL_CFLAGS_BASE): Add DEBUGINFOD_CFLAGS.
	(CLIBS): Add DEBUGINFOD_LIBS.