CMake: Fix bad generation of "revision_data.h" not including MYSQL_EXECUTABLE.

* was caused through invoking "FindMySQL" after using ${MYSQL_EXECUTABLE}.
* better re-run CMake if you are affected by the mysql exe wasn't found issue.

(cherry picked from commit 45a60c2d22)

Conflicts:
	CMakeLists.txt

CMakeLists.txt

@@ -59,6 +59,7 @@ find_package(PCHSupport)
 find_package(OpenSSL REQUIRED)
 find_package(Threads REQUIRED)
 find_package(ZMQ REQUIRED)
+find_package(MySQL)
 
 include(ConfigureBoost)
 

src/server/database/CMakeLists.txt

@@ -8,7 +8,9 @@
 # WITHOUT ANY WARRANTY, to the extent permitted by law; without even the
 # implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 
-find_package(MySQL REQUIRED)
+if (NOT MYSQL_FOUND)
+  message(SEND_ERROR "MySQL wasn't found on your system but it's required to build the servers!")
+endif()
 
 if( USE_COREPCH )
   include_directories(${CMAKE_CURRENT_BINARY_DIR})