GHC configure step can fail when CC=clang

In our internal build of GHC from source, we are building it on Linux with Clang. Unfortunately, this results in an error on ghc-8.4.3:

$ ./configure CC=clang CC_STAGE0=clang --prefix=... --with-ghc=...
...
checking version of gcc... configure: error: Need at least gcc version 4.4 (4.7+ recommended)

Looking in the code, it seems that the logic is wrong: it calls $CC -v and parses the output unconditionally, whether or not the compiler is actually GCC: https://github.com/ghc/ghc/blob/ce7a1c4ae4c93f2d0d3d7a0b573ddd876fc855c2/aclocal.m4#L1245 In our case, the test fails since the compiler is a build of Clang that doesn't produce a useful version number.

I also realized that on macOS (where gcc *is* clang), this test succeeds accidentally, since the output of gcc -v is:

Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 10.0.0 (clang-1000.10.44.2)
Target: x86_64-apple-darwin17.7.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
judahjacobson-macbookpro:~ judahjacobson$ gcc -v
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 10.0.0 (clang-1000.10.44.2)
Target: x86_64-apple-darwin17.7.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture