pre-compile all.h, saves quite some build time

pre-compile all.h, saves quite some build time

before:

$ time CC=clang make -j16
CC=clang make -j16  6,04s user 0,86s system 468% cpu 1,471 total
CC=clang make -j16  6,05s user 0,87s system 468% cpu 1,477 total
CC=clang make -j16  6,15s user 0,86s system 464% cpu 1,510 total
CC=clang make -j16  6,05s user 0,93s system 467% cpu 1,493 total
CC=clang make -j16  6,10s user 0,84s system 461% cpu 1,507 total

$ time CC=gcc make -j16
CC=gcc make -j16  9,91s user 1,43s system 508% cpu 2,231 total
CC=gcc make -j16  10,02s user 1,37s system 500% cpu 2,275 total
CC=gcc make -j16  9,80s user 1,60s system 507% cpu 2,245 total
CC=gcc make -j16  10,02s user 1,44s system 506% cpu 2,264 total
CC=gcc make -j16  9,99s user 1,45s system 505% cpu 2,261 total

after:

$ time CC=clang make -j16
CC=clang make -j16  3,41s user 0,83s system 375% cpu 1,131 total
CC=clang make -j16  3,29s user 0,90s system 373% cpu 1,122 total
CC=clang make -j16  3,35s user 0,77s system 369% cpu 1,116 total
CC=clang make -j16  3,36s user 0,78s system 374% cpu 1,105 total
CC=clang make -j16  3,46s user 0,75s system 373% cpu 1,126 total

$ time CC=gcc make -j16
CC=gcc make -j16  10,74s user 1,44s system 494% cpu 2,462 total
CC=gcc make -j16  10,68s user 1,54s system 497% cpu 2,453 total
CC=gcc make -j16  10,60s user 1,60s system 488% cpu 2,499 total
CC=gcc make -j16  10,63s user 1,51s system 485% cpu 2,502 total
CC=gcc make -j16  10,70s user 1,51s system 497% cpu 2,453 total

Therefore, we enable pre-compiled headers only when CC=clang.