doc: specify protobuf as optional in build docs

Author: fanquake

doc/build-osx.md

@@ -19,7 +19,7 @@ Then install [Homebrew](https://brew.sh).
 
 ## Dependencies
 ```shell
-brew install automake berkeley-db4 libtool boost miniupnpc openssl pkg-config protobuf python qt libevent qrencode
+brew install automake berkeley-db4 libtool boost miniupnpc openssl pkg-config python qt libevent qrencode
 ```
 
 See [dependencies.md](dependencies.md) for a complete overview.

doc/build-unix.md

@@ -112,12 +112,16 @@ To build without GUI pass `--without-gui`.
 
 To build with Qt 5 you need the following:
 
-    sudo apt-get install libqt5gui5 libqt5core5a libqt5dbus5 qttools5-dev qttools5-dev-tools libprotobuf-dev protobuf-compiler
+    sudo apt-get install libqt5gui5 libqt5core5a libqt5dbus5 qttools5-dev qttools5-dev-tools
 
 libqrencode (optional) can be installed with:
 
     sudo apt-get install libqrencode-dev
 
+protobuf (optional) can be installed with:
+
+    sudo apt-get install libprotobuf-dev protobuf-compiler
+
 Once these are installed, they will be found by configure and a bitcoin-qt executable will be
 built by default.
 
@@ -140,12 +144,16 @@ ZMQ dependencies (provides ZMQ API):
 
 To build with Qt 5 you need the following:
 
-    sudo dnf install qt5-qttools-devel qt5-qtbase-devel protobuf-devel
+    sudo dnf install qt5-qttools-devel qt5-qtbase-devel
 
 libqrencode (optional) can be installed with:
 
     sudo dnf install qrencode-devel
 
+protobuf (optional) can be installed with:
+
+    sudo dnf install protobuf-devel
+
 Notes
 -----
 The release is built with GCC and then "strip bitcoind" to strip the debug