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/INSTALLpc.txt b/src/INSTALLpc.txt
index 1c50dd1..43d024a 100644
--- a/src/INSTALLpc.txt
+++ b/src/INSTALLpc.txt
@@ -296,9 +296,10 @@
 
     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: