Update Makefile-common.in to work with mingw

Fix /bin/sh: C:Strawberryperlbinperl.exe: command not found errors with mingw build without (hopefully) breaking other builds.

Author: ronaldxs

tools/build/Makefile-common.in

@@ -98,6 +98,7 @@ CHMOD         = $(PERL) -MExtUtils::Command -e chmod
 CP            = $(PERL) -MExtUtils::Command -e cp
 RM_RF         = $(PERL) -MExtUtils::Command -e rm_rf
 RM_F          = $(PERL) -MExtUtils::Command -e rm_f
+SHELL         = @shell@
 
 SYSROOT         = @sysroot@
 PREFIX          = @prefix@