Our libtool needs bash to work properly.
Patch unbreaks build when /bin/sh points to dash:

    Entering subdirectory liblber
    /bin/sh ../../libtool  --mode=compile x86_64-pc-linux-gnu-gcc -O0 -D_GNU_SOURCE -I../../include -I../../include   -I/usr/include/db4.8 -DLDAP_CONNECTIONLESS   -DLBER_LIBRARY -c assert.c
    ../../build/mkversion -v "2.4.28" liblber.la > version.c
    /bin/sh ../../libtool  --mode=compile x86_64-pc-linux-gnu-gcc -O0 -D_GNU_SOURCE -I../../include -I../../include   -I/usr/include/db4.8 -DLDAP_CONNECTIONLESS   -DLBER_LIBRARY -c decode.c
    eval: 1: base_compile+= x86_64-pc-linux-gnu-gcc: not found
    eval: 1: base_compile+= -O0: not found
    eval: 1: base_compile+= -D_GNU_SOURCE: not found
    eval: 1: base_compile+= x86_64-pc-linux-gnu-gcc: not found
    ...
    make[2]: *** [decode.lo] Error 1
diff --git a/build/top.mk b/build/top.mk
index 6fea488..ea324e3 100644
--- a/build/top.mk
+++ b/build/top.mk
@@ -20,7 +20,7 @@ VERSION= @VERSION@
 RELEASEDATE= @OPENLDAP_RELEASE_DATE@
 
 @SET_MAKE@
-SHELL = /bin/sh
+SHELL = @SHELL@
 
 top_builddir = @top_builddir@