Improve checks for pkg-config and fall back to gpgme-config

#166 changed from `pkg-config`
to `gpgme-config`, but if `PKG_CONFIG_PATH` is not set properly, the C
extension library will quietly build without the right linker
dependencies.

To fix this, we now check for the existence of `gpgme.pc` in
`PKG_CONFIG_PATH` and attempt to fall back to `gpgme-config` if it
does not exist.

Closes #167

ext/gpgme/extconf.rb

@@ -37,13 +37,16 @@
 end
 
 if arg_config('--use-system-libraries', ENV['RUBY_GPGME_USE_SYSTEM_LIBRARIES'])
- unless find_executable('pkg-config')
- $stderr.puts("pkg-config not found")
+ if find_executable('pkg-config') && system('pkg-config gpgme --exists')
+ $CFLAGS += ' ' << `pkg-config --cflags gpgme`.chomp
+ $libs += ' ' << `pkg-config --libs gpgme`.chomp
+ elsif find_executable('gpgme-config')
+ $CFLAGS += ' ' << `gpgme-config --cflags`.chomp
+ $libs += ' ' << `gpgme-config --libs`.chomp
+ else
+ $stderr.puts("pkg-config with gpgme.pc and gpgme-config not found")
  exit(1)
  end
-
- $CFLAGS += ' ' << `pkg-config --cflags gpgme`.chomp
- $libs += ' ' << `pkg-config --libs gpgme`.chomp
 else
  message <<-'EOS'
 ************************************************************************