Ensure that a system has `manpath` before invoking it.

Closes #531, fixes #557.

nvm.sh

@@ -836,20 +836,22 @@ nvm() {
       PATH=`nvm_strip_path "$PATH" "/bin"`
       # Prepend current version
       PATH=`nvm_prepend_path "$PATH" "$NVM_VERSION_DIR/bin"`
-      if [ -z "$MANPATH" ]; then
-        MANPATH=$(manpath)
+      if nvm_has manpath; then
+        if [ -z "$MANPATH" ]; then
+          MANPATH=$(manpath)
+        fi
+        # Strip other version from MANPATH
+        MANPATH=`nvm_strip_path "$MANPATH" "/share/man"`
+        # Prepend current version
+        MANPATH=`nvm_prepend_path "$MANPATH" "$NVM_VERSION_DIR/share/man"`
+        export MANPATH
       fi
-      # Strip other version from MANPATH
-      MANPATH=`nvm_strip_path "$MANPATH" "/share/man"`
-      # Prepend current version
-      MANPATH=`nvm_prepend_path "$MANPATH" "$NVM_VERSION_DIR/share/man"`
       # Strip other version from NODE_PATH
       NODE_PATH=`nvm_strip_path "$NODE_PATH" "/lib/node_modules"`
       # Prepend current version
       NODE_PATH=`nvm_prepend_path "$NODE_PATH" "$NVM_VERSION_DIR/lib/node_modules"`
       export PATH
       hash -r
-      export MANPATH
       export NODE_PATH
       export NVM_PATH="$NVM_VERSION_DIR/lib/node"
       export NVM_BIN="$NVM_VERSION_DIR/bin"