Use pkg_config method to set CFLAGS LDFLAGS correctly

[tompng]

In my new mac(M2), rake compile failed.

After changing extconf.rb like this, rake compile succeed.

+ $CFLAGS = "#{`pkg-config --cflags vterm`} -Werror".delete("\n")
+ $LDFLAGS = `pkg-config --libs vterm`
  create_makefile('vterm')
- $CFLAGS = "#{`pkg-config --cflags vterm`} -Werror"
- $LDFLAGS = `pkg-config --libs vterm`

These are the problems.

create_makefile('vterm')
# Setting global variable after creating makefile has no effect.
$CFLAGS = "#{`pkg-config --cflags vterm`} -Werror" # Output of pkg-config has trailing \n, need to remove it
$LDFLAGS = `pkg-config --libs vterm`

Fortunately, we don't have to set $CFLAGS, $LDFLAGS and other global variables manually. pkg_config('vterm') will do it.

[st0012]

This fixes my installation problem 👍