Document possible use of -fopenmp

Author: eddelbuettel

ChangeLog

@@ -8,6 +8,9 @@
 	* src/RcppExports.cpp: Regenerated
 	* R/RcppExports.R: Idem
 
+	* src/Makevars: Document possible use of '-fopenmp'
+	* src/Makevars.win: Idem
+
 2023-04-18  Dirk Eddelbuettel  <edd@debian.org>
 
 	* README.md: Use app.codecov.io as base for codecov link

src/Makevars

@@ -1,11 +1,11 @@
 ## -*- mode: makefile; -*-
 
+## One could add '-fopenmp' here (and below) for multithreaded operations
 PKG_CXXFLAGS = -I../inst/include
 
-## With Rcpp 0.11.0 and later, we no longer need to set PKG_LIBS for
-## Rcpp as there is no user-facing library. 
+## One could add '-fopenmp' here (and above) for multithreaded operations
 PKG_LIBS = $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS)
 
-## With R 3.1.0 or later, you can uncomment the following line to tell R to 
-## enable compilation with C++11 (where available)
-#USE_CXX1X = 
+## We are not enabling multithreaded operations by default because this would be
+## a change in behaviour that would like create trouble for packages using RcppEigen
+## as CRAN tests of those packages may seen use of more than two cores and protest

src/Makevars.win

@@ -1,11 +1,11 @@
 ## -*- mode: makefile; -*-
 
+## One could add '-fopenmp' here (and below) for multithreaded operations
 PKG_CXXFLAGS = -I../inst/include
 
-## With Rcpp 0.11.0 and later, we no longer need to set PKG_LIBS for
-## Rcpp as there is no user-facing library. 
+## One could add '-fopenmp' here (and above) for multithreaded operations
 PKG_LIBS = $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS)
 
-## With R 3.1.0 or later, you can uncomment the following line to tell R to 
-## enable compilation with C++11 (where available)
-#USE_CXX1X = 
+## We are not enabling multithreaded operations by default because this would be
+## a change in behaviour that would like create trouble for packages using RcppEigen
+## as CRAN tests of those packages may seen use of more than two cores and protest