Revert to install the included libmodbus as opposed to the distro one

There were changes to libmodbus api that breaks compatibility with current OpenPLC code. To keep things working it is better to use the included libmodbus version, even if it is older.

background_installer.sh

@@ -43,7 +43,7 @@ function linux_install_deps {
         $1 apt-get update
         $1 apt-get install -y build-essential pkg-config bison flex autoconf \
                               automake libtool make git \
-                              sqlite3 cmake curl python3 python3-venv libmodbus-dev
+                              sqlite3 cmake curl python3 python3-venv
     else
         fail "Unsupported linux distro."
     fi
@@ -151,8 +151,6 @@ function disable_opendnp3 {
 }
 
 function install_libmodbus {
-    [ -r /usr/include/modbus/modbus.h ] && return 0
-
     echo "[LIBMODBUS]"
     cd "$OPENPLC_DIR/utils/libmodbus_src"
     ./autogen.sh