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)

src/INSTALLpc.txt

@@ -296,9 +296,10 @@ Go to the source directory of Vim, then execute the make command.  E.g.:
 
     CC=clang
     CXX=clang++
-    make -f Make_ming.mak
+    # To build Vim without the GUI support
     make -f Make_ming.mak GUI=no
-    make -f Make_ming.mak GUI=yes
+    # To build Vim with the GUI support
+    make -f Make_ming.mak GUI=yes XPM=no
 
 To build Vim with the address sanitizer (ASAN), execute the following command:
 

src/Make_cyg_ming.mak

@@ -217,10 +217,12 @@ MKDIR = mkdir
 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)

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    5101,
 /**/
     5100,
 /**/