fix(build): check features before mangling CFLAGS

Yet another autotools fix: If any warning/devel CFLAGS would invoke
warnings in headers used to detect dependencies, those dependencies will
fail with a highly misleading/confusing error message:

> configure: Found .git directory.  Adding -Werror to CFLAGS.
> checking if running on EC2 instance... yes
> checking if want AWS platform optimizations... yes
> checking for Libfabric 1.18.0 or later... no
> configure: error: On AWS platforms, Libfabric 1.18.0 or later is required

ie: it is not that Libfabric 1.18 was not found, it was that its headers
produced warnings. Fix this by resolving all dependencies before
modifying CFLAGS.

stack-info: PR: aws#589, branch: aws-nslick/stack/31

configure.ac

@@ -104,6 +104,16 @@ CHECK_PKG_HWLOC([],
 CHECK_PKG_VALGRIND()
 CHECK_VAR_REDZONE()
 
+NCCL_OFI_PLATFORM="none"
+AS_IF([test "${NCCL_OFI_PLATFORM}" = "none"], [AX_CHECK_PLATFORM_AWS()])
+
+AC_SUBST([NCCL_NET_OFI_DISTCHCK_CONFIGURE_FLAGS])
+
+AS_IF([test "${valgrind_enabled}" = "1" -a "${enable_asan}" = "yes"],
+      [AC_MSG_ERROR([Enabling ASAN and valgrind at the same time is not permitted])])
+
+CHECK_ENABLE_MEMFD_CREATE()
+
 # do we want our tests?
 CHECK_PKG_MPI([found_mpi="yes"], [found_mpi="no"])
 
@@ -148,16 +158,6 @@ AS_IF([test -d "${srcdir}/.git" -a -z "${enable_werror}"],
       [AC_MSG_NOTICE([Adding ${werror_flags} to CFLAGS.])
        CFLAGS="${CFLAGS} ${werror_flags}"])
 
-NCCL_OFI_PLATFORM="none"
-AS_IF([test "${NCCL_OFI_PLATFORM}" = "none"], [AX_CHECK_PLATFORM_AWS()])
-
-AC_SUBST([NCCL_NET_OFI_DISTCHCK_CONFIGURE_FLAGS])
-
-AS_IF([test "${valgrind_enabled}" = "1" -a "${enable_asan}" = "yes"],
-      [AC_MSG_ERROR([Enabling ASAN and valgrind at the same time is not permitted])])
-
-CHECK_ENABLE_MEMFD_CREATE()
-
 AC_CONFIG_FILES([Makefile
                  include/Makefile
                  src/Makefile