[doc] minor improvements to build-manuals.sh

doc/build-manuals.sh

 #! /usr/bin/env bash
 
+# GNU parallel needs to be installed
+if ! command -v parallel >/dev/null 2>/dev/null; then
+    echo "parallel is required"
+    exit 99
+fi
+
+# latexmk needs to be installed
+if ! command -v latexmk >/dev/null 2>/dev/null; then
+    echo "latexmk is required"
+    exit 99
+fi
+
 cd $(dirname $0)
 
 usage () {
@@ -20,7 +32,7 @@ fi
 set -e
 
 if [ ! -e acsl ]; then
-    echo "error: 'acsl' not in doc; clone git@github.com:acsl-language/acsl.git"
+    echo "error: 'acsl' not in doc; try: git clone git@github.com:acsl-language/acsl.git"
     exit 1
 fi
 
@@ -32,7 +44,7 @@ ACSL_SUFFIX=$(grep acslversion acsl/version.tex | sed 's/.*{\([^{}\\]*\).*/\1/')
 EACSL_SUFFIX=$(grep 'newcommand{\\eacsllangversion' ../src/plugins/e-acsl/doc/refman/main.tex | sed 's/.*{\([^{}\\]*\).*/\1/')
 # sanity check
 if [ "$EACSL_SUFFIX" = "" ]; then
-    echo "error: could not retrive E-ACSL version from ../src/plugins/e-acsl/doc/refman/main.tex"
+    echo "error: could not retrieve E-ACSL version from ../src/plugins/e-acsl/doc/refman/main.tex"
     exit 1
 fi