Merge #1241: build: Improve SECP_TRY_APPEND_DEFAULT_CFLAGS macro

3addb4c build: Improve `SECP_TRY_APPEND_DEFAULT_CFLAGS` macro (Hennadii Stepanov)

Pull request description:

ACKs for top commit:
  real-or-random:
    utACK 3addb4c
  jonasnick:
    ACK 3addb4c

Tree-SHA512: 918d906570d82be9354fba72bb55d50b8f661cf7cd4404dc244deb489c2bca95b3942ae8af830873ba825dc8ddc68b99c973fc984ff13fdd1f6668f412ca56a3

Author: jonasnick

configure.ac

@@ -93,11 +93,14 @@ esac
 AC_DEFUN([SECP_TRY_APPEND_DEFAULT_CFLAGS], [
     # GCC and compatible (incl. clang)
     if test "x$GCC" = "xyes"; then
-      # Try to append -Werror=unknown-warning-option to CFLAGS temporarily. Otherwise clang will
-      # not error out if it gets unknown warning flags and the checks here will always succeed
-      # no matter if clang knows the flag or not.
+      # Try to append -Werror to CFLAGS temporarily. Otherwise checks for some unsupported
+      # flags will succeed.
+      # Note that failure to append -Werror does not necessarily mean that -Werror is not
+      # supported. The compiler may already be warning about something unrelated, for example
+      # about some path issue. If that is the case, -Werror cannot be used because all
+      # of those warnings would be turned into errors.
       SECP_TRY_APPEND_DEFAULT_CFLAGS_saved_CFLAGS="$CFLAGS"
-      SECP_TRY_APPEND_CFLAGS([-Werror=unknown-warning-option], CFLAGS)
+      SECP_TRY_APPEND_CFLAGS([-Werror], CFLAGS)
 
       SECP_TRY_APPEND_CFLAGS([-std=c89 -pedantic -Wno-long-long -Wnested-externs -Wshadow -Wstrict-prototypes -Wundef], $1) # GCC >= 3.0, -Wlong-long is implied by -pedantic.
       SECP_TRY_APPEND_CFLAGS([-Wno-overlength-strings], $1) # GCC >= 4.2, -Woverlength-strings is implied by -pedantic.