g++, gcc: page update (#7821)

* Update placeholders:
- use "path/to"
- join list of placeholders in gcc

* Suggest language standard in code example

Co-authored-by: Muhammad Falak R Wani <falakreyaz@gmail.com>

Co-authored-by: Muhammad Falak R Wani <falakreyaz@gmail.com>

pages/common/g++.md

@@ -6,16 +6,16 @@
 
 - Compile a source code file into an executable binary:
 
-`g++ {{source.cpp}} -o {{output_executable}}`
+`g++ {{path/to/source.cpp}} -o {{path/to/output_executable}}`
 
 - Display (almost) all errors and warnings:
 
-`g++ {{source.cpp}} -Wall -o {{output_executable}}`
+`g++ {{path/to/source.cpp}} -Wall -o {{path/to/output_executable}}`
 
-- Choose a language standard to compile for(C++98/C++11/C++14/C++17):
+- Choose a language standard to compile for (C++98/C++11/C++14/C++17):
 
-`g++ {{source.cpp}} -std={{language_standard}} -o {{output_executable}}`
+`g++ {{path/to/source.cpp}} -std={{c++98|c++11|c++14|c++17}} -o {{path/to/output_executable}}`
 
 - Include libraries located at a different path than the source file:
 
-`g++ {{source.cpp}} -o {{output_executable}} -I{{header_path}} -L{{library_path}} -l{{library_name}}`
+`g++ {{path/to/source.cpp}} -o {{path/to/output_executable}} -I{{path/to/header}} -L{{path/to/library}} -l{{library_name}}`