[haesbaert]

configure tested CC with test -x, which only accepts a path to one executable. A CC with a launcher or driver flags in front of the compiler, the convention autoconf and make have always honoured, fails that test. configure then path-searched the whole string, found nothing, and went on to gcc from PATH without a word:

$ CC="ccache /opt/.../musl-clang" ./configure
Finding suitable compiler........success [/usr/bin/gcc]

In a cross build that is the wrong compiler, the wrong headers and the wrong instruction set, and nothing downstream can tell. We shipped a static musl binary whose ck objects came from the host's gcc at x86-64-v3, and it died with SIGILL on a Nehalem-class VMware guest at the first ck_hs probe.

Resolve the first word of CC, directly or on PATH, and keep the rest. If CC is set and does not resolve, fail with the value in the message. The cc-then-gcc search stays for a CC that was never set.

CC=<wrapper>              success [<wrapper>]
CC="ccache <wrapper>"     success, ck.build carries the launcher
CC="bogus <wrapper>"      failed  [bogus <wrapper>: not found]
CC=                       success [/usr/bin/cc]

Every later use of CC is unquoted in command position, so the multi-word value needs no other change. Checked under bash and dash.