kconfig: refactor Makefile to reduce process forks

Refactor Makefile and use read-file macro. For Make >= 4.2, it can read
out a file by using the built-in function.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

kbuild: Allow kernel installation packaging to override pkg-config

Add HOSTPKG_CONFIG to allow tooling that builds the kernel to override
what pkg-config and parameters are used.

Signed-off-by: Chun-Tse Shao <ctshao@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

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: 4ab3b80159d4 ("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>

kconfig: refactor GTK+ package checks for building gconf

Refactor the package checks for gconf in the same way as for qconf.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>