qt: fix mysql module build

Closes #2371

Bug 2371 was a Qt compilation issue when MySQL support was enabled,
because the Qt configuration tests were looking at host MySQL
libraries.

Even though I couldn't reproduce the bug with MySQL libraries
installed on my host, there was clearly a problem since
-I/usr/include/mysql and -L/usr/lib/mysql were part of the compilation
flags when building those tests.

This was caused by the fact that /usr/bin/mysql_config was used
instead of $(STAGING_DIR)/usr/bin/mysql_config. We fix this by passing
the path to the correct mysql_config script using the -mysql_config Qt
./configure argument. Since this mysql_config script returns the
correct flags for libraries and headers path, the hardcoded -I and -L
flag in qt.mk are no longer needed.

With this in place, Qt is no longer compiled with -I/usr/include/mysql
and -L/usr/lib/mysql, which presumably should fix the problem reported
in bug 2371.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

CHANGES

@@ -22,6 +22,7 @@
 	#561:  ltp-testsuite failed to install
 	#1651: Build fail caused by ccache in module-init-tools
 	#1735: [PATCH] mplayer: convert to autotools infrastructure
+	#2371: QT MYSQL Module does not build when MySQL installed on the host
 	#2905: Qt: Speed up compilation, if gui-module isn't selected
 	#2965: Broken linkage to xkbcomp (blocking X server startup)
 	#2983: xlib_libX11 build failed

package/qt/qt.mk

@@ -283,8 +283,7 @@ ifeq ($(BR2_PACKAGE_QT_IBASE),y)
 QT_CONFIGURE+= -qt-sql-ibase
 endif
 ifeq ($(BR2_PACKAGE_QT_MYSQL),y)
-QT_CONFIGURE+= -qt-sql-mysql -I $(STAGING_DIR)/usr/include/mysql -L $(STAGING_DIR)/usr/lib/mysql
-
+QT_CONFIGURE+= -qt-sql-mysql -mysql_config $(STAGING_DIR)/usr/bin/mysql_config
 QT_DEP_LIBS+=mysql_client
 endif
 ifeq ($(BR2_PACKAGE_QT_ODBC),y)