cmake: add libcurlu/libcurltool for unit tests

Add a `libcurlu`/`libcurltool` static library that is compiled only for
unit tests. We use `EXCLUDE_FROM_ALL` to make sure that they're not
built by default, they're only built if unit tests are built.

These libraries allow us to compile every unit test with CMake.

Closes #11446

src/CMakeLists.txt

@@ -76,6 +76,14 @@ add_executable(
   ALIAS ${EXE_NAME}
   )
 
+add_library(
+  curltool # special libcurltool library just for unittests
+  STATIC
+  EXCLUDE_FROM_ALL
+  ${CURL_CFILES} ${CURLX_CFILES} ${CURL_HFILES}
+)
+target_compile_definitions(curltool PUBLIC UNITTESTS CURL_STATICLIB)
+
 if(CURL_HAS_LTO)
   set_target_properties(${EXE_NAME} PROPERTIES
     INTERPROCEDURAL_OPTIMIZATION_RELEASE TRUE