Merge #16586: doc: Mention other ways to conserve memory on compilation

fa7789f doc: Mention other ways to conserve memory on compilation (MarcoFalke)

Pull request description:

  Related to:

  *  Building requires >1GB memory #6658

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Tree-SHA512: 828593de9cfa9f9027c6c8e97abe95e3fad9f2ff50e6512808a8950de4a1f9ea901e724edfb96d7119224c3e38a136e60cb798b34ef682c585a7951e02124a3a

Author: fanquake

doc/build-unix.md

@@ -61,6 +61,14 @@ tuned to conserve memory with additional CXXFLAGS:
 
     ./configure CXXFLAGS="--param ggc-min-expand=1 --param ggc-min-heapsize=32768"
 
+Alternatively, or in addition, debugging information can be skipped for compilation. The default compile flags are
+`-g -O2`, and can be changed with:
+
+    ./configure CXXFLAGS="-O2"
+
+Finally, clang (often less resource hungry) can be used instead of gcc, which is used by default:
+
+    ./configure CXX=clang++ CC=clang
 
 ## Linux Distribution Specific Instructions