Fixed .gitignore to ignore libgsqlw.pc
Made internal functions static.
Do not export internal structures.

Declare internal structures with hidden visibility for compilers that
support this.
Fixed configure.ac for aclocal/automake-1.13.

Fixed warning reported by `autoreconf -Wall`:
configure.ac:4: warning: 'AM_CONFIG_HEADER': this macro is obsolete.
configure.ac:4: You should use the 'AC_CONFIG_HEADERS' macro instead.
Updated configure.ac with autoupdate-2.69

This fixes the following `autoreconf-2.69 -Wall` warnings:
configure.ac:10: warning: The macro `AC_PROG_LIBTOOL' is obsolete.
configure.ac:10: You should run autoupdate.
...
configure.ac:37: warning: The macro `AC_HELP_STRING' is obsolete.
configure.ac:37: You should run autoupdate.
...
configure.ac:156: warning: AC_OUTPUT should be used without arguments.
configure.ac:156: You should run autoupdate.
Fixed autoscan warnings.

This fixes the following autoscan warnings:
configure.ac: warning: missing AC_CHECK_FUNCS([memset]) wanted by: gsqlw-mysql.c:272
configure.ac: warning: missing AC_CHECK_FUNCS([strchr]) wanted by: gsqlw-mysql.c:87
configure.ac: warning: missing AC_CHECK_HEADERS([stdlib.h]) wanted by: gsqlw-pgsql.c:20
configure.ac: warning: missing AC_CHECK_HEADERS([string.h]) wanted by: gsqlw-pgsql.c:21
configure.ac: warning: missing AC_CHECK_HEADERS([unistd.h]) wanted by: gsqlw-test.c:22
configure.ac: warning: missing AC_PROG_INSTALL wanted by: autogen.sh:9
configure.ac: warning: missing AC_TYPE_SIZE_T wanted by: gsqlw-mysql.c:260
Removed unused C++ support from configure.ac.
Added m4 directory for libtoolize to be happy.

This fixes the following warning during autogen.sh run:
libtoolize: Consider adding `AC_CONFIG_MACRO_DIR([m4])' to configure.ac and
libtoolize: rerunning libtoolize, to keep the correct libtool macros in-tree.
libtoolize: Consider adding `-I m4' to ACLOCAL_AMFLAGS in Makefile.am.
configure.ac rework and cleanup.

configure.ac changes:
 - remove multiple sqlite3 checking and hardcoding of SQLITE_LIBS;
 - use standard PKG_CHECK_EXISTS() macro instead of invoking pkg-config
   directly;
 - add error message if sqlite3 is enabled but not present;
 - define MYSQL_CFLAGS and MYSQL_LIBS which were used in Makefile.am but
   never defined;
 - don't pollute generic CFLAGS and LDFLAGS with flags for mysql -- use
   dedicated MYSQL_CFLAGS and MYSQL_LIBS for that;
 - remove duplicated mysql ldflags inclusion -- this is not needed under
   normal circumstances;
 - use AS_HELP_STRING() for --with-mysql option;
 - don't check for mysql_config if both --disable-mysql and --with-mysql
   options are present;
 - use standard AC_PATH_PROG() macro for obtaining full paths for
   mysql_config and pg_config where possible;
 - removed unused HAVE_PG_CONFIG_H, HAVE_PQEXECPARAMS and HAVE_PQPREPARE macros
   with corresponding AC_CHECK_LIB() calls;
 - simplify and fix pg_config executability check not to ignore just
   discovered PG_CONFIG variable;
 - remove use of AM_CONDITIONAL() for simple configure-only conditional
   code;
 - replace `test -x ... -a test -f ...` with a more portable
   AS_EXECUTABLE_P();
 - replace if/then/else/fi constructs with a more portable AS_IF();
 - replace case/esac construct with a more portable AS_CASE();
 - allow --enable-* to accept "auto" value.