fix make's impurity on /bin/sh

This is important when using tooling like BEAR to generate
compilation database since the used glibc version needs to match
for LD_PRELOAD to work. It might be also beneficial when building
on systems other than NixOS with nix develop since /bin/sh might
be not bash (which is what all nix devs use for testing).
This fix is not perfect because Makefile.config.in itself is
also build with make but strictly better than the status quo.

Makefile.config.in

@@ -19,6 +19,7 @@ LIBLZMA_LIBS = @LIBLZMA_LIBS@
 OPENSSL_LIBS = @OPENSSL_LIBS@
 PACKAGE_NAME = @PACKAGE_NAME@
 PACKAGE_VERSION = @PACKAGE_VERSION@
+SHELL = @bash@
 SODIUM_LIBS = @SODIUM_LIBS@
 SQLITE3_LIBS = @SQLITE3_LIBS@
 bash = @bash@