build_docs: conditionally install mermaid

Author: sdarwin

build_docs/linuxdocs.sh

@@ -217,8 +217,9 @@ if [ "$skippackagesoption" != "yes" ]; then
         node --version
         npm --version
         npm install gulp-cli@2.3.0
-        npm install @mermaid-js/mermaid-cli@10.5.1
-
+        if grep -r mermaid "$BOOST_SRC_FOLDER/doc/"; then
+            npm install @mermaid-js/mermaid-cli@10.5.1
+        fi
 
     fi
     if [ "$typeoption" = "cppalv1" ]; then

build_docs/macosdocs.sh

@@ -267,7 +267,9 @@ if [ "$skippackagesoption" != "yes" ]; then
         node --version
         npm --version
         npm install gulp-cli@2.3.0
-        npm install @mermaid-js/mermaid-cli@10.5.1
+        if grep -r mermaid "$BOOST_SRC_FOLDER/doc/"; then
+            npm install @mermaid-js/mermaid-cli@10.5.1
+        fi
     fi
 
     brew install ruby

build_docs/windowsdocs.ps1

@@ -417,7 +417,11 @@ if ( -Not ${skip-packages} ) {
         }
 
         npm install gulp-cli@2.3.0
-        npm install @mermaid-js/mermaid-cli@10.5.1
+
+        # need to check this code:
+        if (Get-ChildItem -Path "$BOOST_SRC_FOLDER/doc/" -Recurse -File | Select-String -Pattern "mermaid" -Quiet) {
+            npm install @mermaid-js/mermaid-cli@10.5.1
+        }
 
     }