[config] crash if no C++ compiler is found

Summary: Useful to know that g++/clang++ is missing.

Reviewed By: mbouaziz

Differential Revision: D5256561

fbshipit-source-id: d93ee75

configure.ac

@@ -128,6 +128,9 @@ AC_PROG_MKDIR_P
 if test "x$enable_c_analyzers" = "xyes"; then
   AC_PROG_CPP
   AC_PROG_CXX(clang++ g++)
+  # AC_PROG_CXX doesn't set CXX to "no" in case of failure (I have no words...); from the manual:
+  # "If none of those checks succeed, then as a last resort set CXX to g++. "
+  AS_IF([command -v "$CXX" > /dev/null], [], [AC_MSG_ERROR([no C++ compiler found])])
 
   dnl clang wants either clang version >= 3.1 or gcc version >= 4.7.2 to
   dnl compile itself