Add check whether pkg-config was installed

[Watson1978]

We can build and install capng_c even if pkg-config is not installed.
However, it does not work because it does not link to libcap-ng properly.

Actually, the install log indicates that libcap-ng can't be found.
Ref. checking for pkg-config for libcap-ng... not found line with following install log.

So, this patch will abort install process if pkg-config is not installed.

# rake
mkdir -p tmp/x86_64-linux/capng/3.4.2
cd tmp/x86_64-linux/capng/3.4.2
/root/.rbenv/versions/3.4.2/bin/ruby -I. ../../../../ext/capng/extconf.rb
checking for pkg-config for libcap-ng... not found
checking for CAPNG_SELECT_AMBIENT in cap-ng.h... no
checking for CAPNG_SELECT_ALL in cap-ng.h... no
checking for CAPNG_AMBIENT in cap-ng.h... no
checking for CAPNG_INIT_SUPP_GRP in cap-ng.h... yes
checking for rb_sym2str() in ruby.h... yes
checking for rb_io_descriptor() in ruby.h... yes
checking for capng_get_caps_fd() in cap-ng.h... no
creating Makefile
cd -
cd tmp/x86_64-linux/capng/3.4.2
/usr/bin/gmake
compiling ../../../../ext/capng/capability.c
compiling ../../../../ext/capng/capability_info.c
compiling ../../../../ext/capng/capng.c
compiling ../../../../ext/capng/enum-action.c
compiling ../../../../ext/capng/enum-flags.c
compiling ../../../../ext/capng/enum-result.c
compiling ../../../../ext/capng/enum-select.c
compiling ../../../../ext/capng/enum-type.c
compiling ../../../../ext/capng/enum.c
compiling ../../../../ext/capng/print.c
compiling ../../../../ext/capng/state.c
compiling ../../../../ext/capng/utils.c
linking shared-object capng/capng.so
cd -
mkdir -p tmp/x86_64-linux/stage/lib/capng
/usr/bin/gmake install sitearchdir=../../../../lib/capng sitelibdir=../../../../lib/capng target_prefix=
/usr/bin/install -c -m 0755 capng.so ../../../../lib/capng
cp tmp/x86_64-linux/capng/3.4.2/capng.so tmp/x86_64-linux/stage/lib/capng/capng.so
Loaded suite /root/.rbenv/versions/3.4.2/lib/ruby/gems/3.4.0/gems/rake-13.2.1/lib/rake/rake_test_loader
Started
/root/.rbenv/versions/3.4.2/bin/ruby: symbol lookup error: /root/capng_c/lib/capng/capng.so: undefined symbol: capng_clear
rake aborted!
Command failed with status (127)

[Watson1978]

If pkg-config was not installed, this patch will abort with following message:

# rake
mkdir -p tmp/x86_64-linux/capng/3.4.2
cd tmp/x86_64-linux/capng/3.4.2
/root/.rbenv/versions/3.4.2/bin/ruby -I. ../../../../ext/capng/extconf.rb
checking for pkg-config... no
*** ../../../../ext/capng/extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
	--with-opt-dir
	--without-opt-dir
	--with-opt-include=${opt-dir}/include
	--without-opt-include
	--with-opt-lib=${opt-dir}/lib
	--without-opt-lib
	--with-make-prog
	--without-make-prog
	--srcdir=../../../../ext/capng
	--curdir
	--ruby=/root/.rbenv/versions/3.4.2/bin/$(RUBY_BASE_NAME)
	--with-capng-dir
	--without-capng-dir
	--with-capng-include=${capng-dir}/include
	--without-capng-include
	--with-capng-lib=${capng-dir}/lib
	--without-capng-lib
../../../../ext/capng/extconf.rb:24:in '<main>': Require pkg-config due to install capng_c gem. Please install pkg-config. (RuntimeError)
rake aborted!
Command failed with status (1): [/root/.rbenv/versions/3.4.2/bin/ruby -I. ../../../../ext/capng/extconf.rb]

Tasks: TOP => default => compile => compile:x86_64-linux => compile:capng:x86_64-linux => copy:capng:x86_64-linux:3.4.2 => tmp/x86_64-linux/capng/3.4.2/capng.so => tmp/x86_64-linux/capng/3.4.2/Makefile
(See full trace by running task with --trace)

[daipom]

Thanks!