Fix the Makefile on Solaris

If SHELL is set `make` fails on Solaris with:
mksh: Fatal error: Could not load Shell from `sh': Bad file number
Remove it so it builds.

Configure.pl

@@ -144,7 +144,8 @@
     $config{'makefile-timing'} = $options{'makefile-timing'};
     $config{'stagestats'} = '--stagestats' if $options{'makefile-timing'};
     $config{'cpsep'} = $win ? ';' : ':';
-    $config{'shell'} = $win ? 'cmd' : 'sh';
+    my $shell = $^O eq 'MSWin32' ? 'cmd' : 'sh';
+    $config{'shell'} = $^O eq 'solaris' ? '' : "SHELL   = $shell";
     $config{'runner_suffix'} = $win ? '.bat' : '';
 
     my $make = 'make';

tools/build/Makefile-common-macros.in

@@ -9,7 +9,7 @@ CP      = @cp@
 RM_F    = @rm_f@
 RM_RF   = @rm_rf@
 TEST_F  = @test_f@
-SHELL   = @shell@
+@shell@
 
 SYSROOT= @sysroot@
 SDKROOT= @sdkroot@