Do no consider warnings fatal in CPU Jitter for LTO build (#2769)

At the moment, CPU Jitter source code needs to disable all optimizations.
This means we can't inherit FORTITY_SOURCE and we therefore undefine
the flag through U_FORTIFY_SOURCE. If one were to use annobin as a plugin
and, simultaneously use LTO, annobin will emit a warning if FORTIFY_SOURCE
is undefined. This is fatal if all warnings are treated as errors.

The correct signal to use for below is annobin. But since we know that
annobin is not necessarily being enabled through -fplugin, match with LTO
and disable turning warnings into errors. This is also kinda fragile; LTO
could be defined through other means similar to annobin. But it's what we
have to work with atm.

Author: torben-hansen

third_party/jitterentropy/CMakeLists.txt

@@ -32,6 +32,22 @@ else()
         # https://gcc.gnu.org/wiki/NewWconversion.
         set(JITTER_COMPILE_FLAGS "${JITTER_COMPILE_FLAGS} -Wconversion")
     endif()
+
+    # See t/P319995887.
+    # At the moment, CPU Jitter source code needs to disable all optimizations.
+    # This means we can't inherit FORTITY_SOURCE and we therefore undefine
+    # the flag through U_FORTIFY_SOURCE. If one were to use annobin as a plugin
+    # and, simultaneously use LTO, annobin will emit a warning if FORTIFY_SOURCE
+    # is undefined. This is fatal if all warnings are treated as errors.
+    #
+    # The correct signal to use for below is annobin. But since we know that
+    # annobin is not necessarily being enabled through -fplugin, match with LTO
+    # and disable turning warnings into errors. This is also kinda fragile; LTO
+    # could be defined through other means similar to annobin. But it's what we
+    # have to work with atm.
+    if ("lto" MATCHES "${CMAKE_CFLAGS}")
+        set(JITTER_COMPILE_FLAGS "${JITTER_COMPILE_FLAGS} -Wno-error")
+    endif()
 endif()
 
 if(BORINGSSL_PREFIX)