Xcode 12 and -Werror=implicit-function-declaration

[martelletto]

Hi,

Xcode 12 (released 2020-09-16) enabled -Werror=implicit-function-declaration by default:

https://developer.apple.com/documentation/xcode-release-notes/xcode-12-release-notes

This positively confuses configure.ac:

checking compiler and flags for sanity... no
configure: error: *** compiler cannot create working executables, check config.log ***

With that particular test fixed, configure runs to completion, but there are other occurrences:

conftest.c:71:3: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
conftest.c:113:2: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
conftest.c:231:15: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
conftest.c:247:2: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
conftest.c:252:2: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
conftest.c:252:7: error: implicitly declaring library function 'strcmp' with type 'int (const char *, const char *)' [-Werror,-Wimplicit-function-declaration]
conftest.c:253:2: error: implicit declaration of function 'unlink' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
conftest.c:250:35: error: implicitly declaring library function '_exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
conftest.c:267:13: error: implicit declaration of function 'fork' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
conftest.c:268:9: error: implicit declaration of function 'getppid' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
conftest.c:269:3: error: implicit declaration of function 'sleep' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
conftest.c:370:3: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
conftest.c:382:1: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
conftest.c:382:1: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
conftest.c:371:27: error: implicitly declaring library function 'printf' with type 'int (const char *, ...)' [-Werror,-Wimplicit-function-declaration]
conftest.c:376:42: error: implicitly declaring library function 'printf' with type 'int (const char *, ...)' [-Werror,-Wimplicit-function-declaration]
conftest.c:376:23: error: implicitly declaring library function 'printf' with type 'int (const char *, ...)' [-Werror,-Wimplicit-function-declaration]
conftest.c:398:3: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
conftest.c:406:3: error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]

This is an attempt to fix configure.ac so that the relevant header files are always included.

-p.