Makefile: Add space behind 'install -m'

There are install shell scripts around, often derived from X11 like
contained in glibc or Gash sources, which require to separate -m from
its value. Therefore add a space behind 'install -m' in the Makefile.

Update for clang

Clang also removed K&R support so define IMPLICIT_INT.
Fix clang warning in lib/bt-log.c

about CONST_WANTED & VT_NONCONST

- integrate CONST_WANTED with nocode_wanted
- expr_const() checks VT_NONCONST

produce a more informative error message when parsing _Complex