Always use /bin/sh shell in the build

Closes: #817971

debian/rules

@@ -21,7 +21,18 @@ DEB_DESTDIR=$(CURDIR)/debian/tmp
 include /usr/share/cdbs/1/class/autotools.mk
 
 # Adds extra options when calling the configure script:
-DEB_CONFIGURE_EXTRA_FLAGS := --disable-shared --without-libcrack --mandir=/usr/share/man --with-libpam --enable-shadowgrp --enable-man --disable-account-tools-setuid --with-group-name-max-length=32 --without-acl --without-attr --without-tcb
+DEB_CONFIGURE_EXTRA_FLAGS := --disable-shared \
+	--without-libcrack \
+	--mandir=/usr/share/man \
+	--with-libpam \
+	--enable-shadowgrp \
+	--enable-man \
+	--disable-account-tools-setuid \
+	--with-group-name-max-length=32 \
+	--without-acl \
+	--without-attr \
+	--without-tcb \
+	 SHELL=/bin/sh
 ifneq ($(DEB_BUILD_GNU_TYPE),$(DEB_HOST_GNU_TYPE))
   DEB_CONFIGURE_EXTRA_FLAGS += --host=$(DEB_HOST_GNU_TYPE)
 endif