Support GRADLE_WRAPPER override in CMake build (similar to aeron project)

[tkohlman]

The aeron project's CMakeList.txt allows the local gradlew script to be overridden by an environment variable. This is useful if you want to specify a different gradle version and/or avoid a download during a build. This feature would be useful in the SBE project as well.

Suggest to add the following in CMakeLists.txt:

if (NOT DEFINED GRADLE_WRAPPER)
    if (MSVC)
        set(GRADLE_WRAPPER "gradlew.bat" CACHE INTERNAL "Location of the Gradle wrapper script")
    else ()
        set(GRADLE_WRAPPER "./gradlew" CACHE INTERNAL "Location of the Gradle wrapper script")
    endif ()
endif ()

And use it in this custom command (also CMakeLists.txt):

add_custom_command(OUTPUT ${SBE_JAR}
    COMMAND ${GRADLE_WRAPPER} assemble -x javadoc --console=plain
    DEPENDS ${SBE_SOURCES}
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
    COMMENT "Generating SBE jar"
)