cmake: more small tidy-ups

- prefix local variables with underscore and convert to lowercase.
- list variables accepted by `libcurl.pc` and `curl-config` templates.
- quote more string literals.

Follow-up to 919394ee64 #14450
Closes #14462

tests/CMakeLists.txt

@@ -27,7 +27,7 @@ find_program(TEST_NGHTTPX "nghttpx")
 if(NOT TEST_NGHTTPX)
   set(TEST_NGHTTPX "nghttpx")
 endif()
-# TEST_NGHTTPX
+# Consumed variables: TEST_NGHTTPX
 configure_file("config.in" "${CMAKE_CURRENT_BINARY_DIR}/config" @ONLY)
 
 add_custom_target(testdeps)

tests/http/CMakeLists.txt

@@ -47,5 +47,5 @@ if(NOT HTTPD_NGHTTPX)
   set(HTTPD_NGHTTPX "")
 endif()
 
-# APXS, HTTPD, APACHECTL, HTTPD_NGHTTPX, CADDY, VSFTPD
+# Consumed variables: APACHECTL, APXS, CADDY, HTTPD, HTTPD_NGHTTPX, VSFTPD
 configure_file("config.ini.in" "${CMAKE_CURRENT_BINARY_DIR}/config.ini" @ONLY)