[CMake] Can't get CMake to use an import library generated from a MODULE target

[Sam Habiel]

Rather than email one of the folks at Kitware, I decided to join the
CMake mailing list... Hello all for those who remember me.

I apologize for the long email.

A few of us have been trying to port GT.M
(https://www.fisglobal.com/Solutions/Services/Database-Engine) to
Cygwin, 1. to get it running on Windows 2. as an exercise to prepare
for harder ports, the Raspberry Pi being the holy grail here. That's
the background.

The problem we are having is familiar to many; but not initially to
me. See the heading "Shared libraries with Windows/MinGW" in this
article: http://gernotklingler.com/blog/creating-using-shared-libraries-different-compilers-different-operating-systems/.

CMakeLists.txt:
https://github.com/shabiel/fis-gtm/blob/cygwin/CMakeLists.txt

The error:
[ 88%] Linking C shared module plugin/cyggtmcrypt_gcrypt_AES256CFB.dll
CMakeFiles/libgtmcrypt_gcrypt_AES256CFB.so.dir/sr_unix/gtmcrypt_util.c.o:
In function `gc_load_gtmshr_symbols':
/home/sam/fis-gtm-cygwin/sr_unix/gtmcrypt_util.c:100: undefined
reference to `gtm_malloc'
/home/sam/fis-gtm-cygwin/sr_unix/gtmcrypt_util.c:101: undefined
reference to `gtm_free'
collect2: error: ld returned 1 exit status
CMakeFiles/libgtmcrypt_gcrypt_AES256CFB.so.dir/build.make:202: recipe
for target 'plugin/cyggtmcrypt_gcrypt_AES256CFB.dll' failed
make[2]: *** [plugin/cyggtmcrypt_gcrypt_AES256CFB.dll] Error 1
CMakeFiles/Makefile2:891: recipe for target
'CMakeFiles/libgtmcrypt_gcrypt_AES256CFB.so.dir/all' failed
make[1]: *** [CMakeFiles/libgtmcrypt_gcrypt_AES256CFB.so.dir/all] Error 2
Makefile:116: recipe for target 'all' failed
make: *** [all] Error 2

Here's the link command (I turned CMAKE_VERBOSE_MAKEFILE=ON):
/usr/bin/cc   -march=i586 -fsigned-char -Wmissing-prototypes
-Wreturn-type -Wpointer-sign -fno-omit-frame-pointer -g -DDEBUG
-shared -Wl,--enable-auto-import -o
plugin/cyggtmcrypt_gcrypt_AES256CFB.dll
-Wl,--major-image-version,0,--minor-image-version,0
CMakeFiles/libgtmcrypt_gcrypt_AES256CFB.so.dir/sr_unix/gtmcrypt_ref.c.o
CMakeFiles/libgtmcrypt_gcrypt_AES256CFB.so.dir/sr_unix/gtmcrypt_pk_ref.c.o
CMakeFiles/libgtmcrypt_gcrypt_AES256CFB.so.dir/sr_unix/gtmcrypt_dbk_ref.c.o
CMakeFiles/libgtmcrypt_gcrypt_AES256CFB.so.dir/sr_unix/gtmcrypt_sym_ref.c.o
CMakeFiles/libgtmcrypt_gcrypt_AES256CFB.so.dir/sr_unix/gtmcrypt_util.c.o
-lgpg-error -lgpgme -lgcrypt /usr/local/lib/libconfig.dll.a

What is missing is that there needs to be either a link to
cyggtmshr.dll, or a way for CMake to know to generate a
libgtmshr_import.lib and add it to the link command.

So here is what I tried:
1. Using the GenerateExportHeader functionality of CMake on libgtmshr.
If I do that, it doesn't seem to generate the import library; but it
sure does generate an export header, which I don't need. Here it is:
include(GenerateExportHeader)
generate_export_header(libgtmshr
    BASE_NAME libgtmshr
    EXPORT_MACRO_NAME libgtmshr_EXPORTS
    EXPORT_FILE_NAME  libgtmshr_EXPORTS.h
    STATIC_DEFINE SHARED_EXPORTS_BUILT_AS_STATIC)

2. Explicitly adding a libgtmshr as a dependency on line
https://github.com/shabiel/fis-gtm/blob/cygwin/CMakeLists.txt#L531,
but only for Cygwin. I get this error message:
CMake Error at CMakeLists.txt:542 (target_link_libraries):
  Target "libgtmshr" of type MODULE_LIBRARY may not be linked into another
  target.  One may link only to STATIC or SHARED libraries, or to executables
  with the ENABLE_EXPORTS property set.

The GT.M developer familiar with CMake suggested me to use LINK_FLAGS
and have CMake use the file by us hardcoding it in; which I am okay
doing, but I thought I would check to see if there is an alternative
way of doing this.

I also tried the suggestions here:
"https://blog.kitware.com/create-dlls-on-windows-without-declspec-using-new-cmake-export-all-feature/"
to no avail.

Last, but not least:

sam at horus ~/fis-gtm-cygwin
$ cmake --version
cmake version 3.4.3

CMake suite maintained and supported by Kitware (kitware.com/cmake).

sam at horus ~/fis-gtm-cygwin
$ uname -a
CYGWIN_NT-10.0-WOW horus 2.2.0(0.289/5/3) 2015-08-03 12:49 i686 Cygwin

-- 
Sam Habiel, Pharm.D.
VISTA Expertise Network