retain PKG_* user-supplied env values

[mattdowle]

Closes #4664

Aside: I removed the quotes around "${CC}" "${CFLAGS}" on line 88 because I got an incidental error gcc -std=gnu99: not found when I was testing. Removing those quotes made that error go away.

Shame we can't use += in Makevars. It would be nice to know which system's make do not support +=, just Solaris? Note that WRE note 70 states

[+=] was apparently introduced in SunOS 4, and is available elsewhere provided it is surrounded by spaces.

I don't know how SunOS 4 relates to Solaris. And does "elsewhere" mean "everywhere else"? Perhaps WRE is close to allowing +=.

[codecov]

Codecov Report

Merging #4735 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4735   +/-   ##
=======================================
  Coverage   99.44%   99.44%           
=======================================
  Files          73       73           
  Lines       14539    14539           
=======================================
  Hits        14458    14458           
  Misses         81       81           

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