Refactor build_doxygen.sh to use set -euo pipefail

This refactoring should make it easier to read the script, and less
likely that anyone accidentally breaks the script in future.

Author: John Nonweiler

scripts/build_doxygen.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+set -euo pipefail
+
 DOXYGEN_VERSION=$1
 
 if [ $# -ne 1 ]
@@ -8,7 +10,7 @@ then
   exit 1
 fi
 
-mkdir -p doxygen/build \
-&& wget http://ftp.stack.nl/pub/users/dimitri/doxygen-${DOXYGEN_VERSION}.src.tar.gz -O- | tar -xz --strip-components=1 --directory doxygen \
-&& ( cd doxygen/build && cmake .. ) \
-&& make -j4 -C doxygen/build
+mkdir -p doxygen/build
+wget http://ftp.stack.nl/pub/users/dimitri/doxygen-${DOXYGEN_VERSION}.src.tar.gz -O- | tar -xz --strip-components=1 --directory doxygen
+( cd doxygen/build && cmake .. )
+make -j4 -C doxygen/build