autogen.sh: add check for pkg-config being present

Since a ./configure switch was added to enable bash-completion
installation support, pkg-config is now required during the build.

This adds a check in autogen.sh to ensure pkg-config is installed.

Closes: resurrecting-open-source-projects#19

autogen.sh

@@ -2,6 +2,7 @@
 
 # autogen.sh with clean option
 # Copyright 2016 Joao Eriberto Mota Filho <eriberto@eriberto.pro.br>
+# Copyright 2023 David da Silva Polverari <david.polverari@gmail.com>
 #
 # This file is under BSD-3-Clause license.
 #
@@ -47,6 +48,13 @@ then
     exit 0
 fi
 
+env pkg-config --version > /dev/null 2>&1
+if [ $? -ne 0 ]
+then
+    echo "pkg-config is missing. Please install it and run $0 again."
+    exit 1
+fi
+
 # Do autoreconf
 autoreconf -i \
    && { echo " "; \