add checks for libs & headers required by IBM DB2

ext/odbc/config.m4

@@ -194,6 +194,15 @@ AC_ARG_WITH(ibm-db2,
       ODBC_INCDIR=$withval/include
       ODBC_LIBDIR=$withval/$PHP_LIBDIR
     fi
+
+    if ! test -f "$ODBC_INCDIR/sqlcli1.h"; then
+      AC_MSG_ERROR([IBM DB2 header files not found])
+    fi
+
+    if ! test -f "$ODBC_LIBDIR/libdb2.so"; then
+      AC_MSG_ERROR([IBM DB2 required libraries not found])
+    fi
+	
     ODBC_INCLUDE=-I$ODBC_INCDIR
     ODBC_LFLAGS=-L$ODBC_LIBDIR
     ODBC_TYPE=db2