package/ccache: always disable developer mode

Ccache CMake build system has some conditions that automatically
enables a developer mode (sets CCACHE_DEV_MODE=ON). See [1].

More specifically, if CCACHE_DEV_MODE is unset AND the environment
variable "CI" is set, CCACHE_DEV_MODE is set to "ON".

This situation can happen when Buildroot builds are executed in
Jenkins jobs, for example. Since Buildroot does not set
CCACHE_DEV_MODE and Jenkins sets the "CI" environment variable,
this ccache developer mode can be enabled in an unexpected way
for the Buildroot user. For example, it happened that a Jenkins build
breaks, while the build with the same configuration in the user
session is working.

One of the effects of enabling this ccache developer mode, is to treat
compiler warnings as errors, see [3]. This can lead to build error,
depending on the ccache version and the host compiler being used.

This behavior can be reproduced and observed, with commands:

    cat > .config <<EOF
    BR2_aarch64=y
    BR2_TOOLCHAIN_EXTERNAL=y
    BR2_CCACHE=y
    BR2_PACKAGE_BUSYBOX=y
    EOF
    make olddefconfig

    make clean
    make host-ccache

Outputs:

    ...
    -- Ccache version: 4.9.1
    -- Ccache dev mode: OFF
    -- Setting CMAKE_BUILD_TYPE to Release as none was specified.
    ...

Whereas:

    make clean
    CI=true make host-ccache

Outputs:

    ...
    -- Ccache version: 4.9.1
    -- Ccache dev mode: ON
    -- Setting CMAKE_BUILD_TYPE to Debug as none was specified.
    ...

For a failure example: on Fedora 40 with host gcc 14.1.1, Buildroot at
tag 2024.02 has ccache 4.8.2. Host ccache can fail, when building with
"CI=true make host-ccache" with output:

    /buildroot/output/build/host-ccache-4.8.2/src/third_party/fmt/core.h:3119:44:   in 'constexpr' expansion of 'fmt::v8::make_format_args<>(args#0, args#1)'
    /buildroot/output/build/host-ccache-4.8.2/src/third_party/fmt/core.h:1706:15: error: possibly dangling reference to a temporary [-Werror=dangling-reference]
     1706 |   const auto& arg = arg_mapper<Context>().map(std::forward<T>(val));
          |               ^~~

This commit sets CCACHE_DEV_MODE=OFF to make the ccache behavior more
deterministic in Buildroot, independently of being used in a CI tool
or not.

[1] https://github.com/ccache/ccache/blob/v4.9.1/CMakeLists.txt#L56
[2] https://github.com/jenkinsci/jenkins/blob/jenkins-2.459/core/src/main/java/jenkins/model/CoreEnvironmentContributor.java#L43
[3] https://github.com/ccache/ccache/blob/v4.9.1/cmake/StandardWarnings.cmake#L5

Reported-by: Xavier Roumegue <xroumegue@gmail.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 197be7ed87)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/ccache/ccache.mk

@@ -15,6 +15,7 @@ HOST_CCACHE_DEPENDENCIES = host-hiredis host-zstd
 HOST_CCACHE_CONF_OPTS += \
 	-UCMAKE_C_COMPILER_LAUNCHER \
 	-UCMAKE_CXX_COMPILER_LAUNCHER \
+	-DCCACHE_DEV_MODE=OFF \
 	-DZSTD_FROM_INTERNET=OFF \
 	-DHIREDIS_FROM_INTERNET=OFF \
 	-DENABLE_TESTING=OFF