[RFC PATCH 3/5] CI: add a linux-BUILD-vars job

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Add a linux-BUILD-vars job, in a preceding commit we fixed a bug that
would have been spotted by testing under NO_CURL=Y.

This CI job attempts to unset various settings in config.mak.uname and
the Makefile, so that we'll stress our fallbacks and conditionally
compiled code as much as possible.

If there is a missing setting here that we can enable under
"ubuntu-latest" the omission isn't intentional, this list came from a
quick skimming of the relevant parts of the Makefile and
config.mak.uname.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 .github/workflows/main.yml |  3 +++
 ci/run-build-and-tests.sh  | 30 ++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 3fa88b78b6d..25263c6b17a 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -229,6 +229,9 @@ jobs:
             cc: gcc
             os: ubuntu
             cc_package: gcc-8
+          - jobname: linux-BUILD-vars
+            cc: gcc
+            os: ubuntu
             pool: ubuntu-latest
           - jobname: osx-clang
             cc: clang
diff --git a/ci/run-build-and-tests.sh b/ci/run-build-and-tests.sh
index 8ebff425967..786285c5071 100755
--- a/ci/run-build-and-tests.sh
+++ b/ci/run-build-and-tests.sh
@@ -31,6 +31,36 @@ linux-TEST-vars)
 	export GIT_TEST_WRITE_REV_INDEX=1
 	export GIT_TEST_CHECKOUT_WORKERS=2
 	;;
+linux-BUILD-vars)
+	export NO_CURL=Y
+	export NO_PTHREADS=Y
+	export NO_GETTEXT=Y
+
+	# Undo settings in config.mak.uname
+	export HAVE_ALLOCA_H=
+
+	# Various compat/ fallbacks, with "FAIL" omitted if faking it
+	# doesn't work on Linux.
+	export NO_REGEX=Y
+	export NO_MEMMEM=Y
+	export INTERNAL_QSORT=Y
+	export SNPRINTF_RETURNS_BOGUS=Y
+	export FREAD_READS_DIRECTORIES=Y
+	export OPEN_RETURNS_EINTR=Y
+	export NO_HSTRERROR= # compat/hstrerror.c FAIL
+	export NO_POLL=Y
+	export NO_STRCASESTR=Y
+	export NO_STRTOUMAX=Y
+	export NO_SETENV=Y
+	export NO_UNSETENV=Y
+	export NO_MMAP=Y
+	export NO_PREAD=Y
+	export NEEDS_MODE_TRANSLATION= # compat/stat.c FAIL
+	export NO_IPV6=Y
+	export NO_INET_NTOP=Y
+	export NO_INET_PTON=Y
+	export NO_UNIX_SOCKETS=Y
+	;;
 linux-clang)
 	export GIT_TEST_DEFAULT_HASH=sha1
 	;;
-- 
2.36.1.1239.gfba91521d90