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: NMeelis Roos <mroos@linux.ee>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: NMeelis Roos <mroos@linux.ee>
Tested-by: NRandy Dunlap <rdunlap@infradead.org>

Currently, the necessary package checks for building qconf is
surrounded by ifeq ($(MAKECMDGOALS),xconfig) ... endif.
Then, Make will restart when .tmp_qtcheck is generated.

To simplify the Makefile, move the scripting to a separate file,
and use filechk.  The shell script is executed everytime xconfig
is run, but it is not a costly script.

In the old code, 'pkg-config --exists' only checked Qt5Core / QtCore,
but the set of necessary packages should be checked.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: NRandy Dunlap <rdunlap@infradead.org>
Acked-by: NRandy Dunlap <rdunlap@infradead.org>
Reviewed-by: NSam Ravnborg <sam@ravnborg.org>