lib: include files using known path

by including headers using "../[header]" when done from C files in
subdirectories, we do not need to specify the lib source dir as an
include path and we reduce the risk of header name collisions with
headers in the SDK using the same file names.

Idea-by: Kai Pastor

Ref: #16949
Closes #16991

src/Makefile.am

@@ -121,7 +121,7 @@ endif
 # Use absolute directory to disable VPATH
 ASCIIPAGE=$(top_builddir)/docs/cmdline-opts/curl.txt
 MKHELP=$(top_srcdir)/src/mkhelp.pl
-HUGE=tool_hugehelp.c
+HUGE=$(builddir)/tool_hugehelp.c
 
 HUGECMD = $(HUGEIT_$(V))
 HUGEIT_0 = @echo "  HUGE    " $@;
@@ -164,7 +164,7 @@ curl_cfiles_gen += $(HUGE)
 curl_hfiles_gen += tool_hugehelp.h
 CLEANFILES += $(HUGE)
 
-CA_EMBED_CSOURCE = tool_ca_embed.c
+CA_EMBED_CSOURCE = $(builddir)/tool_ca_embed.c
 curl_cfiles_gen += $(CA_EMBED_CSOURCE)
 CLEANFILES += $(CA_EMBED_CSOURCE)
 if CURL_CA_EMBED_SET
@@ -184,12 +184,7 @@ CS_ = $(CS_0)
 
 # ignore generated C files since they play by slightly different rules!
 checksrc:
-	$(CHECKSRC)(@PERL@ $(top_srcdir)/scripts/checksrc.pl -D$(srcdir) \
-	  -W$(srcdir)/$(HUGE) \
-	  -W$(srcdir)/$(CA_EMBED_CSOURCE) \
-	  -W$(srcdir)/curltool_unity.c \
-	  -W$(srcdir)/libcurltool_unity.c \
-	  $(srcdir)/*.[ch])
+	$(CHECKSRC)(@PERL@ $(top_srcdir)/scripts/checksrc.pl -D$(srcdir) $(CURL_CFILES) $(CURL_HFILES))
 
 if DEBUGBUILD
 # for debug builds, we scan the sources on all regular make invokes