Fix restoring CFLAGS and LIBS in AX_LIB_LMDB()

Fix restoring CFLAGS and LIBS in AX_LIB_LMDB()

The AX_LIB_LMDB() macro attempts to test the potential LMDB installation
path provided to it by temporarily updating CFLAGS and LIBS, calling
AC_SEARCH_LIBS(), and then restoring CFLAGS and LIBS to their original
values.  However, including certain statements (e.g. "break") in the
arguments provided to the AX_LIB_LMDB() macro may cause an early exit
from it, in which case CFLAGS and LIBS will be left polluted.  Fix by
resetting CFLAGS and LIBS to their original values before executing the
commands provided as AX_LIB_LMDB() arguments.