unify windows mkspecs: update description

Common changes to mingw-w64, ICC on Windows and MSVC toolchains:
- update toolchains description similar to 'gcc-base.conf'.

Change-Id: Ie456c6cec86c0d1c0107ca84a0fa7855666df91e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

mkspecs/common/msvc-desktop.conf

@@ -1,10 +1,14 @@
 #
-# qmake configuration for Microsoft Visual Studio C/C++ Compiler
-# This mkspec is used by the win32-msvc and win32-clang-msvc specs
+# This file is used as a basis for the following compilers:
 #
-
+# - Microsoft C/C++ Optimizing Compiler (all desktop versions)
+# - Intel C++ Compiler on Windows
+# - Clang-cl
+#
+# Baseline:
+#
+# - Visual Studio 2005 (8.0), VC++ 14.0
 #
-# Baseline: Visual Studio 2005 (8.0), VC++ 14.0
 # Version-specific settings go in msvc-version.conf (loaded by default_pre)
 #
 

mkspecs/win32-g++/qmake.conf

@@ -1,7 +1,7 @@
 #
 # qmake configuration for win32-g++
 #
-# Written for MinGW / gcc 4.6 or higher
+# Written for MinGW-w64 / gcc 5.3 or higher
 #
 # Cross compile example for i686-w64-mingw32-g++:
 #   configure -xplatform win32-g++ -device-option CROSS_COMPILE=i686-w64-mingw32-

mkspecs/win32-icc/qmake.conf

@@ -1,16 +1,17 @@
 #
 # qmake configuration for win32-icc
 #
-# Written for Intel C++
+# Written for Intel C++ Compiler on Windows / icl 16.0 or higher
 #
 
-# Use the Visual Studio configuration
+# Use the Microsoft (R) C/C++ Optimizing Compiler configuration,
+# since ICC on Windows pretends to be MSVC
 
 include(../common/msvc-desktop.conf)
 
-# Now override with the Intel compiler settings
+# modifications to msvc-desktop.conf
 
-QMAKE_COMPILER         += intel_icl  # icl pretends to be msvc
+QMAKE_COMPILER         += intel_icl
 
 QMAKE_CC                = icl
 QMAKE_CFLAGS            = -nologo -Zm200 /Qprec /Qwd1744,1738,809,3373

mkspecs/win32-msvc/qmake.conf

@@ -1,7 +1,7 @@
 #
 # qmake configuration for win32-msvc
 #
-# Written for Microsoft Visual C++ (all desktop versions)
+# Written for Microsoft C/C++ Optimizing Compiler (all desktop versions)
 #
 
 include(../common/msvc-desktop.conf)