autotools: enable warnings and cleanup

- enable warnings in autogen script
- drop unused m4/ directory usage
- drop AC_TYPE_SIGNAL:
  C99 guarantees the signal return type to be void
- drop AC_CHECK_FILE of procdir:
  most of the time compilation is done on a different system than htop is run
  and there is a runtime check in place
- improve linux_affinity corss compile logic:
  use fourth argument instead of pre-test

Makefile.am

@@ -1,5 +1,3 @@
-
-ACLOCAL_AMFLAGS = -I m4
 AUTOMAKE_OPTIONS = subdir-objects
 
 bin_PROGRAMS = htop