disable ccache in a friendly way

[lanefu]

should make this the default.. enables in case PRIVATE_CCACHE=yes is set

reasoning per @rpardini:

• ccache has 0 cache hit ratio during any kernel version change (version.h)
• but causes every build to be slower to gather/compare cache
• armbian/build already tries hard to keep your (worktree) cache make-hot during rebuilds/repatching/reconfigs

[rpardini]

this PR is superior

[rpardini]

[adeepn]

• ccache has 0 cache hit ratio during any kernel version change (version.h)

how many .c files do you think include version.h?

[igorpecovnik]

Please add bits to the docs, here:

https://docs.armbian.com/Developer-Guide_Build-Switches/#advanced

[adeepn]

linux kernel 6.13.0-rc7 for linux-meson64 on my notebook (time may float due to the use of a laptop during testing, but the trend is clearly visible)

before:

  Cache size (GiB): 0.0 / 5.0 ( 0.00%)
  Cleanups:          61

first build without ccache

real	16m20,265s
user	226m45,734s
sys	28m8,309s

first build with ccache:

export PATH=/usr/lib/ccache:$PATH
export ARCH=arm64
export CROSS_COMPILE=aarch64-none-linux-gnu-
export REAL_CC=aarch64-none-linux-gnu-gcc
make -j16

real	19m40,902s
user	273m16,985s
sys	35m37,396s

$ ccache -s
Cacheable calls:    19117 / 19336 (98.87%)
  Hits:              3502 / 19117 (18.32%)
    Direct:          3502 /  3502 (100.0%)
    Preprocessed:       0 /  3502 ( 0.00%)
  Misses:           15615 / 19117 (81.68%)
Uncacheable calls:    219 / 19336 ( 1.13%)
Local storage:
  Cache size (GiB):   0.5 /   5.0 (10.80%)
  Hits:              3502 / 19117 (18.32%)
  Misses:           15615 / 19117 (81.68%)

repeatable build with ccache:

real	0m38,227s
user	2m52,498s
sys	3m38,345s

real	0m38,355s
user	3m5,663s
sys	4m3,897s

after version change in Makefile build with ccache:

$ ccache -s
Cacheable calls:    62798 / 63685 (98.61%)
  Hits:             47057 / 62798 (74.93%)
    Direct:         34870 / 47057 (74.10%)
    Preprocessed:   12187 / 47057 (25.90%)
  Misses:           15741 / 62798 (25.07%)
Uncacheable calls:    887 / 63685 ( 1.39%)
Local storage:
  Cache size (GiB):   0.5 /   5.0 (10.84%)
  Hits:             47057 / 62798 (74.93%)
  Misses:           15741 / 62798 (25.07%)

real	7m26,959s
user	97m44,005s
sys	17m41,464s

reset ccache stats:

$ ccache -s
Cacheable calls:    15633 / 15833 (98.74%)
  Hits:             15632 / 15633 (99.99%)
    Direct:         15632 / 15632 (100.0%)
    Preprocessed:       0 / 15632 ( 0.00%)
  Misses:               1 / 15633 ( 0.01%)
Uncacheable calls:    200 / 15833 ( 1.26%)
Local storage:
  Cache size (GiB):   0.5 /   5.0 (10.84%)
  Hits:             15632 / 15633 (99.99%)
  Misses:               1 / 15633 ( 0.01%)

repeatable rebuild with ccache without version change:

real	0m42,210s
user	3m15,798s
sys	4m26,176s

[lanefu]

Please add bits to the docs, here:

https://docs.armbian.com/Developer-Guide_Build-Switches/#advanced

armbian/documentation#603