tests: parametrize "libc++" in tests

Downstream has no `libc++`, but it can run clang tests with `libstdc++`.
Parametrize the tests, leaving the actual library to use on their
respective `main.fmf` files that are easier to overturn in downstream
repos.

tests/llvm-toolchain/main.fmf

@@ -1,12 +1,30 @@
 # TODO REVIEW: better summary
 summary: ""
 test: ./test.sh
-require:
-  - clang
-  - lld
-  - compiler-rt
-  - libcxx-devel
-  - libcxx-static
-  - glibc-devel
-  - glibc-static
-  - gcc
+adjust:
+  - environment+:
+      CXXLIB: "libc++"
+    require:
+      - clang
+      - lld
+      - compiler-rt
+      - libcxx-devel
+      - libcxx-static
+      - glibc-devel
+      - glibc-static
+      - gcc
+    when: "distro == fedora"
+    because: testing against libcxx package in Fedora
+  - environment+:
+      CXXLIB: "libstdc++"
+    require:
+      - clang
+      - lld
+      - compiler-rt
+      - glibc-devel
+      - glibc-static
+      - gcc
+      - libstdc++
+    when: "distro == rhel"
+    because: testing against libstdc++ package in RHEL as libcxx is not shipped with RHEL
+require: []

tests/llvm-toolchain/test.sh

@@ -4,6 +4,12 @@
 
 set pipefail
 
+if [ -z "${CXXLIB:-}" ]; then
+  echo "CXXLIB variable is a required input but it's not specified!"
+  echo "Test metadata should have picked a proper value, depending on distro."
+  exit 1
+fi
+
 # Test compile a C program.
 cat << EOF | \
 	clang -fuse-ld=lld -rtlib=compiler-rt -x c - && \
@@ -18,7 +24,7 @@ EOF
 
 # Test compile a C++ program.
 cat << EOF | \
-	clang++ -x c++ -fuse-ld=lld -rtlib=compiler-rt -stdlib=libc++ - && \
+	clang++ -x c++ -fuse-ld=lld -rtlib=compiler-rt -stdlib="$CXXLIB" - && \
 	./a.out | grep 'Hello World'
 
 #include <iostream>

tests/toolchains/main.fmf

@@ -1,7 +1,21 @@
 # TODO REVIEW: better summary
 summary: ""
 test: ./test.sh
+adjust:
+  - environment+:
+      CXXLIBS: "libc++"
+    require:
+      - clang
+      - libcxx-devel
+    when: "distro == fedora"
+    because: testing against libcxx package in Fedora
+  - environment+:
+      CXXLIBS: "libstdc++"
+    require:
+      - clang
+      - libstdc++
+    when: "distro == rhel"
+    because: testing against libstdc++ package in RHEL as libcxx is not shipped with RHEL
 # TODO REVIEW: are these all requirements? test.sh seems to run quite a lot of stuff, looks like we
 # need more packages from LLVM family.
-require:
-  - clang
+require: []

tests/toolchains/test.sh

@@ -2,6 +2,12 @@
 
 set pipefail
 
+if [ -z "${CXXLIBS:-}" ]; then
+  echo "CXXLIBS variable is a required input but it's not specified!"
+  echo "Test metadata should have picked a proper value, depending on distro."
+  exit 1
+fi
+
 status=0
 
 test_toolchain() {
@@ -25,6 +31,9 @@ test_toolchain() {
             libc++)
                 args="$args -stdlib=$1"
                 ;;
+            libstdc++)
+                args="$args -stdlib=$1"
+                ;;
             lld)
                 args="$args -fuse-ld=$1"
                 ;;
@@ -54,7 +63,7 @@ echo ""
 for compiler in clang clang++; do
     for rtlib in "" compiler-rt; do
         for linker in "" lld; do
-            for cxxlib in "" libc++; do
+            for cxxlib in "" $CXXLIBS; do
                 if [ "$compiler" = "clang" -a -n "$cxxlib" ]; then
                     continue
                 fi