build: tidy up header paths, use srcdir where possible

To improve readability.

Also add more comments on why each is necessary.

Closes #17630

tests/client/CMakeLists.txt

@@ -47,7 +47,7 @@ target_include_directories(${BUNDLE} PRIVATE
   "${PROJECT_BINARY_DIR}/lib"            # for "curl_config.h"
   "${PROJECT_SOURCE_DIR}/lib"            # for "curl_setup.h"
   "${PROJECT_SOURCE_DIR}/lib/curlx"      # for curlx
-  "${CMAKE_CURRENT_SOURCE_DIR}"          # for "first.h"
+  "${CMAKE_CURRENT_SOURCE_DIR}"          # for the generated bundle source to find included test sources
 )
 set_property(TARGET ${BUNDLE} APPEND PROPERTY COMPILE_DEFINITIONS "CURL_NO_OLDIES")
 set_target_properties(${BUNDLE} PROPERTIES OUTPUT_NAME "${BUNDLE}" PROJECT_LABEL "Test ${BUNDLE}" UNITY_BUILD OFF)

tests/client/Makefile.am

@@ -31,6 +31,7 @@ AUTOMAKE_OPTIONS = foreign nostdinc
 # $(top_srcdir)/include is for libcurl's external include files
 # $(top_builddir)/lib is for libcurl's generated lib/curl_config.h file
 # $(top_srcdir)/lib for libcurl's lib/curl_setup.h and other "borrowed" files
+# $(srcdir) for the generated bundle source to find included test sources
 
 AM_CPPFLAGS = -I$(top_srcdir)/include        \
               -I$(top_builddir)/lib          \