build: windows: Eagerly fail on missing openssl or flex

Windows builds will fail if flex, bison or the OpenSSL libraries
are missing. So modify the CMake code to ensure that we fail at
CMake time, rather than during the subsequent build.

CMakeLists.txt

@@ -606,6 +606,12 @@ if(FLB_TLS)
   if(OPENSSL_FOUND)
     FLB_DEFINITION(FLB_HAVE_OPENSSL)
   endif()
+
+  if (FLB_SYSTEM_WINDOWS AND NOT(OPENSSL_FOUND))
+    # win32 builds w/o openssl will fail later so we might as well catch it
+    # early instead.
+    MESSAGE(FATAL_ERROR "OpenSSL required on Windows, see DEVELOPER_GUIDE.md")
+  endif()
 endif()
 
 # Metrics

cmake/windows-setup.cmake

@@ -118,9 +118,10 @@ find_package(FLEX)
 find_package(BISON)
 
 if (NOT (${FLEX_FOUND} AND ${BISON_FOUND}))
-  message(STATUS "flex and bison not found. Disable stream_processor building.")
-  set(FLB_STREAM_PROCESSOR No)
-  set(FLB_RECORD_ACCESSOR  No)
+  # The build will fail later if flex and bison are missing, so there's no
+  # point attempting to continue. There's no test cover for windows builds
+  # without FLB_PARSER anyway.
+  message(FATAL_ERROR "flex and bison not found, see DEVELOPER_GUIDE.md")
 endif()
 
 if (MSVC)