Build: configure: clean tool checking part a bit

• remove determined symbolic references to: . HTML2TXT, POD2MAN (no relevance found) . FOP (no direct use) . SSH, SCP, TAR (not referred symbolically) . MD5 (sic, should have been MD5SUM, introduced with b333c585d; tools/cibsecret.in detects md5sum in run-time; no other relevance found)

• do not attempt to mangle with MAKE variable (as opposed to MAKEFLAGS), since autoconf (if not other autotools) manages that on its own, and attempt to isolate user from make/gmake dichotomy on some systems makes everything only more cryptic compared to transparently requiring "gmake" from the get-go on such systems; systems where "make" implementation doesn't set MAKE variable are covered automatically (with Automake applying AC_PROG_MAKE_SET/@SET_MAKE@ pair of Autoconf)

• simplify AC_PATH_PROGS -> AC_PATH_PROG, and quote the respective arguments properly