MLR: do not use the system blas by default

It may conflict with the application's chosen BLAS, or bring surprising
runtime behavior.

configure.ac

@@ -3004,6 +3004,9 @@ fi
 AC_ARG_ENABLE(mlr, [AS_HELP_STRING([--disable-mlr],
 			[Disable multiple linear regression models])],
 			enable_mlr=$enableval, enable_mlr=$default_enable_mlr)
+AC_ARG_ENABLE(mlr-system-blas, [AS_HELP_STRING([--enable-mlr-system-blas],
+			[Make the multiple linear regression models use the system BLAS instead of min-dgels])],
+			enable_mlr_blas=$enableval, enable_mlr_blas=no)
 
 AC_MSG_CHECKING(whether multiple linear regression models are disabled)
 if test x$enable_mlr = xyes -a "$starpu_windows" != "yes" ; then
@@ -3014,11 +3017,11 @@ if test x$enable_mlr = xyes -a "$starpu_windows" != "yes" ; then
 	if test x$blas_lib = xnone ; then
 	   use_system_lapack=no
 	fi
-	if test x$use_system_lapack = xyes; then
+	if test x$enable_mlr_blas = xyes -a test x$use_system_lapack = xyes; then
 	   	AC_DEFINE(STARPU_MLR_MODEL, [1], [use reflapack library])
 		LDFLAGS="-llapack $LDFLAGS"
 	else
-		if test x$blas_lib = xmkl; then
+		if test x$enable_mlr_blas=xyes -a test x$blas_lib = xmkl; then
 		   	AC_DEFINE(STARPU_MLR_MODEL, [1], [use mkl library])
 		else
 			AC_MSG_CHECKING(whether min-dgels is linked)

doc/doxygen/chapters/370_online_performance_tools.doxy

@@ -375,7 +375,11 @@ parameter are stored in <c>.starpu/sampling/codelets/tmp/</c>
 directory. These files are reused when \ref STARPU_CALIBRATE
 environment variable is set to <c>1</c>, to recompute coefficients
 based on the current, but also on the previous
-executions. Additionally, when multiple linear regression models are
+executions. By default StarPU uses a lightweight dgels implementation, but the
+\ref enable-mlr-system-blas "--enable-mlr-system-blas" configure option can be
+used to make StarPU use a system-provided dgels BLAS.
+
+Additionally, when multiple linear regression models are
 disabled (using \ref disable-mlr "--disable-mlr" configure option) or when the
 <c>model->combinations</c> are not defined, StarPU will still write
 output files into <c>.starpu/sampling/codelets/tmp/</c> to allow

doc/doxygen/chapters/510_configure_options.doxy

@@ -768,6 +768,14 @@ this parameter is 10. Experimental.
 Allow to disable multiple linear regression models (see \ref PerformanceModelExample)
 </dd>
 
+<dt>--enable-mlr-system-blas</dt>
+<dd>
+\anchor enable-mlr-system-blas
+\addindex __configure__--enable-mlr-system-blas
+Allow to make multiple linear regression models use the system-provided BLAS for dgels
+(see \ref PerformanceModelExample)
+</dd>
+
 </dl>
 
 */