cmake: fix curl-config --static-libs

Libraries for `curl-config --static-libs` are required to have the form
-ldl. Assume library paths like /usr/lib/libssl.so and map it to "ssl"
for curl-config.

This removes the confusing messag "Static linking is broken" which was
printed because curl-config --static-libs was disfunctional while the
static libcurl.a is fine.

Fixes curl#841

CMakeLists.txt

@@ -1133,9 +1133,7 @@ set(CURL_CA_BUNDLE          "")
 set(CURLVERSION             "${CURL_VERSION}")
 set(ENABLE_SHARED           "yes")
 if(CURL_STATICLIB)
-  # Broken: LIBCURL_LIBS below; .a lib is not built
-  message(WARNING "Static linking is broken!")
-  set(ENABLE_STATIC         "no")
+  set(ENABLE_STATIC         "yes")
 else()
   set(ENABLE_STATIC         "no")
 endif()
@@ -1144,9 +1142,9 @@ set(includedir              "\${prefix}/include")
 set(LDFLAGS                 "${CMAKE_SHARED_LINKER_FLAGS}")
 set(LIBCURL_LIBS            "")
 set(libdir                  "${CMAKE_INSTALL_PREFIX}/lib")
-# TODO CURL_LIBS also contains absolute paths which don't work with static -l...
 foreach(_lib ${CMAKE_C_IMPLICIT_LINK_LIBRARIES} ${CURL_LIBS})
-  set(LIBCURL_LIBS          "${LIBCURL_LIBS} -l${_lib}")
+  string(REGEX REPLACE "^.*/lib(.+)\\.so$" "\\1" _libname "${_lib}")
+  set(LIBCURL_LIBS          "${LIBCURL_LIBS} -l${_libname}")
 endforeach()
 # "a" (Linux) or "lib" (Windows)
 string(REPLACE "." "" libext "${CMAKE_STATIC_LIBRARY_SUFFIX}")