patch 8.2.5101: MS-Windows with MinGW: $CC may be "cc" instead of "gcc"

Problem:    MS-Windows with MinGW: $CC may be "cc" instead of "gcc".
Solution:   Set $CC if it is not matching "clang". (Yegappan Lakshmanan,
            closes #10578)
diff --git a/src/Make_cyg_ming.mak b/src/Make_cyg_ming.mak
index 61df9a7..9c004fa 100644
--- a/src/Make_cyg_ming.mak
+++ b/src/Make_cyg_ming.mak
@@ -217,10 +217,12 @@
 DIRSLASH = \\
  endif
 endif
-ifeq ($(CC),)
+# set $CC to "gcc" unless it matches "clang"
+ifeq ($(findstring clang,$(CC)),)
 CC := $(CROSS_COMPILE)gcc
 endif
-ifeq ($(CXX),)
+# set $CXX to "g++" unless it matches "clang"
+ifeq ($(findstring clang,$(CXX)),)
 CXX := $(CROSS_COMPILE)g++
 endif
 ifeq ($(UNDER_CYGWIN),yes)