cmake: respect custom `RC` flags and delete `GCC_WINDRES`

Before this patch, `zlib.rc` was compiled using a manual command [1] when
using the MinGW (and MSYS/Cygwin) toolchains. This method ignores
`CMAKE_RC_FLAGS` and offers no other way to pass a custom flag, breaking
the build in cases where a custom `windres` option is required. E.g.
`--target=` or `-I` on some platforms and configuration, in particular
with `llvm-windres`.

This patch deletes the special case for these toolchains and lets CMake
compile the `.rc` file the default way used for all Windows targets.

I'm not entirely sure why this special case was added back in 2011. The
need to pass `-DGCC_WINDRES` is my suspect. We can resolve this much
simpler by adding this line for the targets that require it:
   set(CMAKE_RC_FLAGS "${CMAKE_RC_FLAGS} -DGCC_WINDRES")

But, the `.rc` line protected by `GCC_WINDRES`, these days work just fine
with `windres`. Moreover, that protected line are oboslete flags from the
16-bit era, which for a long time have no effect, as documented here:
<https://docs.microsoft.com/windows/win32/menurc/common-resource-attributes>

So, this patch deletes `GCC_WINDRES` from the project entirely.

[1] dc5a43e

4 files changed, 2 insertions, 31 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index bf7944a..f8c1f13 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -965,26 +965,8 @@ set(ZLIB_GZFILE_SRCS
     gzwrite.c
 )
 
-if(NOT MINGW AND NOT MSYS AND NOT CYGWIN)
-    set(ZLIB_DLL_SRCS
-        win32/zlib${SUFFIX}1.rc # If present will override custom build rule below.
-    )
-endif()
-
-if(MINGW OR MSYS OR CYGWIN)
-    # This gets us DLL resource information when compiling on MinGW.
-    if(NOT CMAKE_RC_COMPILER)
-        set(CMAKE_RC_COMPILER windres.exe)
-    endif()
-
-    add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/zlib1rc.obj
-        COMMAND ${CMAKE_RC_COMPILER}
-            -D GCC_WINDRES
-            -I ${CMAKE_CURRENT_SOURCE_DIR}
-            -I ${CMAKE_CURRENT_BINARY_DIR}
-            -o ${CMAKE_CURRENT_BINARY_DIR}/zlib1rc.obj
-            -i ${CMAKE_CURRENT_SOURCE_DIR}/win32/zlib${SUFFIX}1.rc)
-    set(ZLIB_DLL_SRCS ${CMAKE_CURRENT_BINARY_DIR}/zlib1rc.obj)
+if(NOT DEFINED BUILD_SHARED_LIBS OR BUILD_SHARED_LIBS)
+    set(ZLIB_DLL_SRCS win32/zlib${SUFFIX}1.rc)
 endif()
 
 set(ZLIB_ALL_SRCS ${ZLIB_SRCS} ${ZLIB_ARCH_HDRS} ${ZLIB_ARCH_SRCS} ${ZLIB_DLL_SRCS}
diff --git a/configure b/configure
index a7dfc5b..83b3ead 100755
--- a/configure
+++ b/configure
@@ -402,7 +402,6 @@ if test "$gcc" -eq 1 && ($cc $CFLAGS -c $test.c) >> configure.log 2>&1; then
         LDSHAREDLIBC=""
         DEFFILE='win32/${LIBNAME2}.def'
         RC="${CROSS_PREFIX}windres"
-        RCFLAGS='--define GCC_WINDRES'
         RCOBJS='zlibrc.o'
         STRIP="${CROSS_PREFIX}strip"
         EXE='.exe' ;;
@@ -425,7 +424,6 @@ if test "$gcc" -eq 1 && ($cc $CFLAGS -c $test.c) >> configure.log 2>&1; then
         LDSHAREDLIBC=""
         DEFFILE='win32/${LIBNAME2}.def'
         RC="${CROSS_PREFIX}windres"
-        RCFLAGS='--define GCC_WINDRES'
         RCOBJS='zlibrc.o'
         STRIP="${CROSS_PREFIX}strip"
         EXE='.exe' ;;
@@ -445,7 +443,6 @@ if test "$gcc" -eq 1 && ($cc $CFLAGS -c $test.c) >> configure.log 2>&1; then
         LDSHAREDLIBC=""
         DEFFILE='win32/${LIBNAME2}.def'
         RC="${CROSS_PREFIX}windres"
-        RCFLAGS='--define GCC_WINDRES'
         if [ "$CC" == "mingw32-gcc" ]; then
           case $ARCH in
           i386 | i486 | i586 | i686) RCFLAGS="${RCFLAGS} -F pe-i386";;
diff --git a/win32/zlib-ng1.rc b/win32/zlib-ng1.rc
index b539069..128b56d 100644
--- a/win32/zlib-ng1.rc
+++ b/win32/zlib-ng1.rc
@@ -1,11 +1,7 @@
 #include <winver.h>
 #include "../zlib-ng.h"
 
-#ifdef GCC_WINDRES
 VS_VERSION_INFO		VERSIONINFO
-#else
-VS_VERSION_INFO		VERSIONINFO	MOVEABLE IMPURE LOADONCALL DISCARDABLE
-#endif
   FILEVERSION		ZLIBNG_VER_MAJOR,ZLIBNG_VER_MINOR,ZLIBNG_VER_REVISION,0
   PRODUCTVERSION	ZLIBNG_VER_MAJOR,ZLIBNG_VER_MINOR,ZLIBNG_VER_REVISION,0
   FILEFLAGSMASK		VS_FFI_FILEFLAGSMASK
diff --git a/win32/zlib1.rc b/win32/zlib1.rc
index 11be5f4..39bdcc0 100644
--- a/win32/zlib1.rc
+++ b/win32/zlib1.rc
@@ -1,11 +1,7 @@
 #include <winver.h>
 #include "../zlib.h"
 
-#ifdef GCC_WINDRES
 VS_VERSION_INFO		VERSIONINFO
-#else
-VS_VERSION_INFO		VERSIONINFO	MOVEABLE IMPURE LOADONCALL DISCARDABLE
-#endif
   FILEVERSION		ZLIB_VER_MAJOR,ZLIB_VER_MINOR,ZLIB_VER_REVISION,0
   PRODUCTVERSION	ZLIB_VER_MAJOR,ZLIB_VER_MINOR,ZLIB_VER_REVISION,0
   FILEFLAGSMASK		VS_FFI_FILEFLAGSMASK