patch 8.2.0739: incomplete profiling when exiting because of a dealy signal

Problem:    Incomplete profiling when exiting because of a dealy signal.
Solution:   Call __gcov_flush() if available.
diff --git a/src/Makefile b/src/Makefile
index 6c7ef85..7c2934a 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -686,8 +686,10 @@
 # 7. Open the objects/index.html file in a web browser to view the coverage
 #    information.
 #
-# PROFILE_CFLAGS=-g -O0 -fprofile-arcs -ftest-coverage
 # LDFLAGS=--coverage
+# PROFILE_CFLAGS=-g -O0 -fprofile-arcs -ftest-coverage -DWE_ARE_PROFILING -DUSE_GCOV_FLUSH
+# Alternate flags
+# PROFILE_CFLAGS=-g -O0 --coverage -DWE_ARE_PROFILING -DUSE_GCOV_FLUSH
 
 
 # Uncomment one of the next two lines to compile Vim with the