Add configure help instructions to customize MAGMA

build/configure

@@ -1499,7 +1499,8 @@ Optional Packages:
   --with-sysroot[=DIR]    Search for dependent libraries within DIR (or the
                           compiler's sysroot if not specified).
   --with-lapack           use LAPACK [default=auto]
-  --with-magma            use MAGMA [default=auto]
+  --with-magma[=MAGMA_DIR]
+                          use MAGMA [default=auto]
   --with-include          additional include folders [default=none]
   --with-fflags=flags     FORTRAN compiler flags [default="-std=legacy -O3"]
   --with-ldflags=flags    custom linker flags [default=none]

build/configure.ac

@@ -542,7 +542,7 @@ AC_ARG_WITH(
 
 AC_ARG_WITH(
   [magma],
-  [AS_HELP_STRING([--with-magma], [use MAGMA @<:@default=auto@:>@])],
+  [AS_HELP_STRING([--with-magma[[=MAGMA_DIR]]], [use MAGMA @<:@default=auto@:>@])],
   [
     if test "x$withval" = "xno"; then
       AC_SUBST([MAGMAFLAGS], [""])