improve portability of docs makefile

- detect if /bin/bash is not found and provide a suitable error message
- use $(MAKE) and $(MFLAGS) variables to handle cases where GNU make is not called "make"

doc/Makefile

@@ -1,6 +1,12 @@
 # Makefile for LAMMPS documentation
 
 SHELL          = /bin/bash
+HAS_BASH       = YES
+ifeq (,$(wildcard $(SHELL)))
+OSHELL         := $(SHELL)
+override SHELL = /bin/sh
+HAS_BASH       = NO
+endif
 BUILDDIR       = ${CURDIR}
 RSTDIR         = $(BUILDDIR)/src
 VENV           = $(BUILDDIR)/docenv
@@ -55,6 +61,7 @@ DOXYFILES      = $(shell sed -n -e 's/\#.*$$//' -e '/^ *INPUT \+=/,/^[A-Z_]\+ \+
 # ------------------------------------------
 
 help:
+	@if [ "$(HAS_BASH)" == "NO" ] ; then echo "bash was not found at $(OSHELL)! Please use: $(MAKE) SHELL=/path/to/bash" 1>&2; exit 1; fi
 	@echo "Please use \`make <target>' where <target> is one of"
 	@echo "  html          create HTML doc pages in html dir"
 	@echo "  pdf           create Manual.pdf in this dir"
@@ -87,6 +94,7 @@ $(SPHINXCONFIG)/conf.py: $(SPHINXCONFIG)/conf.py.in
 	    -e 's,@LAMMPS_DOC_DIR@,$(BUILDDIR),g' $< > $@
 
 html: xmlgen $(SPHINXCONFIG)/conf.py $(ANCHORCHECK) $(MATHJAX) $(POLYFILL)
+	@if [ "$(HAS_BASH)" == "NO" ] ; then echo "bash was not found at $(OSHELL)! Please use: $(MAKE) SHELL=/path/to/bash" 1>&2; exit 1; fi
 	@$(MAKE) $(MFLAGS) -C graphviz all
 	@(\
 		. $(VENV)/bin/activate ; env PYTHONWARNINGS= \
@@ -110,6 +118,7 @@ html: xmlgen $(SPHINXCONFIG)/conf.py $(ANCHORCHECK) $(MATHJAX) $(POLYFILL)
 	@echo "Build finished. The HTML pages are in doc/html."
 
 spelling: xmlgen $(VENV) $(SPHINXCONFIG)/false_positives.txt
+	@if [ "$(HAS_BASH)" == "NO" ] ; then echo "bash was not found at $(OSHELL)! Please use: $(MAKE) SHELL=/path/to/bash" 1>&2; exit 1; fi
 	@(\
 		. $(VENV)/bin/activate ; env PYTHONWARNINGS= \
 		cp $(SPHINXCONFIG)/false_positives.txt $(RSTDIR)/ ; env PYTHONWARNINGS= \
@@ -120,6 +129,7 @@ spelling: xmlgen $(VENV) $(SPHINXCONFIG)/false_positives.txt
 	@echo "Spell check finished."
 
 epub: xmlgen $(VENV) $(SPHINXCONFIG)/conf.py $(ANCHORCHECK)
+	@if [ "$(HAS_BASH)" == "NO" ] ; then echo "bash was not found at $(OSHELL)! Please use: $(MAKE) SHELL=/path/to/bash" 1>&2; exit 1; fi
 	@$(MAKE) $(MFLAGS) -C graphviz all
 	@mkdir -p epub/JPG
 	@rm -f LAMMPS.epub
@@ -140,6 +150,7 @@ mobi: epub
 	@echo "Conversion finished. The MOBI manual file is created."
 
 pdf: xmlgen $(VENV) $(SPHINXCONFIG)/conf.py $(ANCHORCHECK)
+	@if [ "$(HAS_BASH)" == "NO" ] ; then echo "bash was not found at $(OSHELL)! Please use: $(MAKE) SHELL=/path/to/bash" 1>&2; exit 1; fi
 	@$(MAKE) $(MFLAGS) -C graphviz all
 	@if [ "$(HAS_PDFLATEX)" == "NO" ] ; then echo "PDFLaTeX was not found! Please check README.md for further instructions" 1>&2; exit 1; fi
 	@(\
@@ -161,9 +172,9 @@ pdf: xmlgen $(VENV) $(SPHINXCONFIG)/conf.py $(ANCHORCHECK)
 		mv tmp.tex LAMMPS.tex && \
 		sed 's/\\end{equation}//g' LAMMPS.tex > tmp.tex && \
 		mv tmp.tex LAMMPS.tex && \
-		make && \
-		make && \
-		make && \
+		$(MAKE) $(MFLAGS) && \
+		$(MAKE) $(MFLAGS) && \
+		$(make) $(MFLAGS) && \
 		mv LAMMPS.pdf ../Manual.pdf && \
 		cd ../;
 	@rm -rf latex/_sources
@@ -211,6 +222,7 @@ doxygen/xml/index.xml : $(VENV) doxygen/Doxyfile $(DOXYFILES)
 # ------------------------------------------
 
 $(VENV):
+	@if [ "$(HAS_BASH)" == "NO" ] ; then echo "bash was not found at $(OSHELL)! Please use: $(MAKE) SHELL=/path/to/bash" 1>&2; exit 1; fi
 	@if [ "$(HAS_PYTHON3)" == "NO" ] ; then echo "python3 was not found! Please see README for further instructions" 1>&2; exit 1; fi
 	@if [ "$(HAS_DOXYGEN)" == "NO" ] ; then echo "doxygen was not found! Please see README for further instructions" 1>&2; exit 1; fi
 	@if [ "$(HAS_VIRTUALENV)" == "NO" ] ; then echo "virtualenv was not found! Please see README for further instructions" 1>&2; exit 1; fi

doc/graphviz/Makefile

@@ -1,6 +1,5 @@
 # Makefile for generating images with graphviz
 #
-SHELL      = /bin/bash
 BUILDDIR   = ${CURDIR}/..
 IMGDIR     = $(BUILDDIR)/src/JPG
 IMGSRC     = $(wildcard *.dot)