bootstrap: Add support for CXX containing flags

Rather than treating the user-provided CXX as a space-separated series of
compilers, treat it as a single command-line fragment which possibly
contains flags.

bootstrap

@@ -1288,14 +1288,17 @@ cmake_cxx_compiler=
 # If CC is set, use that for compiler, otherwise use list of known compilers
 if test -n "${cmake_toolchain}"; then
   eval cmake_cxx_compilers="\${cmake_toolchain_${cmake_toolchain}_CXX}"
-elif test -n "${CXX}"; then
-  cmake_cxx_compilers="${CXX}"
 else
   cmake_cxx_compilers="${CMAKE_KNOWN_CXX_COMPILERS}"
 fi
 
 # Check if C++ compiler works
-TMPFILE=`cmake_tmp_file`
+cmake_cxx_compiler_try_set()
+{
+  test_compiler="$1"
+  test_thread_flags="$2"
+
+  TMPFILE=`cmake_tmp_file`
 echo '
 #include <iostream>
 #include <memory>
@@ -1374,23 +1377,34 @@ int main()
   return 0;
 }
 ' > "${TMPFILE}.cxx"
-for std in 17 14 11; do
-  std_flags="`cmake_extract_standard_flags \"${cmake_toolchain}\" CXX \"${std}\"`"
-  for compiler in ${cmake_cxx_compilers}; do
+  for std in 17 14 11; do
+    std_flags="`cmake_extract_standard_flags \"${cmake_toolchain}\" CXX \"${std}\"`"
     for std_flag in '' $std_flags; do
-      for thread_flag in '' $thread_flags; do
-        echo "Checking whether '${compiler} ${cmake_cxx_flags} ${cmake_ld_flags} ${std_flag} ${thread_flag}' works." >> cmake_bootstrap.log 2>&1
-        if cmake_try_run "${compiler}" "${cmake_cxx_flags} ${cmake_ld_flags} ${std_flag} ${thread_flag}" \
+      for thread_flag in '' $test_thread_flags; do
+        echo "Checking whether '${test_compiler} ${cmake_cxx_flags} ${cmake_ld_flags} ${std_flag} ${thread_flag}' works." >> cmake_bootstrap.log 2>&1
+        if cmake_try_run "${test_compiler}" "${cmake_cxx_flags} ${cmake_ld_flags} ${std_flag} ${thread_flag}" \
           "${TMPFILE}.cxx" >> cmake_bootstrap.log 2>&1; then
-          cmake_cxx_compiler="${compiler}"
+          cmake_cxx_compiler="${test_compiler}"
           cmake_cxx_flags="${cmake_cxx_flags} ${std_flag} ${thread_flag} "
-          break 4
+          rm -f "${TMPFILE}.cxx"
+          return 0
         fi
       done
     done
   done
-done
-rm -f "${TMPFILE}.cxx"
+  rm -f "${TMPFILE}.cxx"
+  return 1
+}
+
+if test -n "${CXX}"; then
+  cmake_cxx_compiler_try_set "${CXX}" "${thread_flags}"
+else
+  for compiler in ${cmake_cxx_compilers}; do
+    if cmake_cxx_compiler_try_set "${compiler}" "${thread_flags}"; then
+      break
+    fi
+  done
+fi
 
 if test -z "${cmake_cxx_compiler}"; then
 cmake_error 7 "Cannot find a C++ compiler that supports both C++11 and the specified C++ flags.