kconfig: do not require pkg-config on make {menu,n}config

Meelis Roos reported a {menu,n}config regression:
 "I have libncurses devel package installed in the default system
  location (as do 99%+ on actual developers probably) and in this
  case, pkg-config is useless.  pkg-config is needed only when
  libraries and headers are installed in non-default locations but
  it is bad to require installation of pkg-config on all the machines
  where make menuconfig would be possibly run."

For {menu,n}config, do not use pkg-config if it is not installed.
For {g,x}config, keep checking pkg-config since we really rely on it
for finding the installation paths of the required packages.

Fixes: 4ab3b801 ("kconfig: check for pkg-config on make {menu,n,g,x}config")
Reported-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Meelis Roos <mroos@linux.ee>
Tested-by: Randy Dunlap <rdunlap@infradead.org>