[build] Allow user to override BASE_CXXFLAGS (in addition to CXXFLAGS)

Also rename OIL_NINJA_VERBOSE -> OILS_CXX_VERBOSE

Example command to compile with cosmoc++:

    OILS_CXX_VERBOSE=1 BASE_CXXFLAGS= _build/oils.sh ~/install/cosmocc/bin/cosmoc++ 2>err.txt

Main error now seems to be not supporting -fexceptions.

Related to issue #1760.

README-native.txt

@@ -1,7 +1,8 @@
 Oils for Unix
 =============
 
-This is a preview of Oil's translation to C++.
+This is the pure C++ tarball for Oils.  (In contrast to the "executable spec",
+it has no CPython code).
 
 To use it, run
 
@@ -25,3 +26,15 @@ Send feedback to:
 
 (TODO: Replace this with INSTALL.txt)
 
+
+## More build configuration
+
+You can pass the compiler and build variant to _build/oils.sh:
+
+    _build/oils.sh ~/install/cosmocc/bin/cosmoc++ dbg
+
+The default values are 'cxx' (c++ system compiler), and 'opt' (optimized build)
+
+You can also override the variables documented at the top of
+build/ninja-rules-cpp.sh (e.g. BASE_CXXFLAGS, CXXFLAGS)
+

build/common.sh

@@ -54,7 +54,15 @@ CXX=${CXX:-'c++'}
 # - TODO(6/22): Disabled invalid-offsetof for now, but we should enable it after
 #   progress on the garbage collector.  It could catch bugs.
 
-BASE_CXXFLAGS='-std=c++11 -Wall -Wno-invalid-offsetof -fno-omit-frame-pointer'
+# Allow user to override both BASE_CXXFLAGS and CXXFLAGS
+# There doesn't seem to be a well-known convention for this.  Similar to this
+# question:
+# - https://stackoverflow.com/questions/51606653/allowing-users-to-override-cflags-cxxflags-and-friends
+
+default_cxx_flags='-std=c++11 -Wall -Wno-invalid-offsetof -fno-omit-frame-pointer'
+
+# note: Use - and not :- so that BASE_CXXFLAGS= works
+BASE_CXXFLAGS=${BASE_CXXFLAGS-$default_cxx_flags}
 
 readonly PY27=Python-2.7.13
 

build/ninja-rules-cpp.sh

@@ -11,9 +11,10 @@
 #   build/ninja-rules-cpp.sh <function name>
 #
 # Env variables:
-#   CXXFLAGS= - additional flags
-#   OIL_NINJA_VERBOSE=1 - show command lines
-#   TIME_TSV_OUT=file - compile_one and link output rows to this TSV file
+#   BASE_CXXFLAGS=        default flags passed to all compiler invocations
+#   CXXFLAGS=             additional flags
+#   OILS_CXX_VERBOSE=1    show compiler command lines
+#   TIME_TSV_OUT=file     compile_one and link output rows to this TSV file
 
 set -o nounset
 set -o errexit
@@ -71,7 +72,13 @@ setglobal_compile_flags() {
   # flags from Ninja/shell respected
   flags="$BASE_CXXFLAGS -I $REPO_ROOT $more_cxx_flags"
 
-  # flags from env respected
+  # Flags from env
+  # Similar to
+  # - GNU make - https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html
+  #   CXXFLAGS "Extra flags to give to the C++ compiler"
+  # - CMake - https://cmake.org/cmake/help/latest/envvar/CXXFLAGS.html 
+  #   "Add default compilation flags to be used when compiling CXX (C++) files."
+
   local env_flags=${CXXFLAGS:-}
   if test -n "$env_flags"; then
     flags="$flags $env_flags"
@@ -268,7 +275,7 @@ compile_one() {
 
   setglobal_cxx $compiler
 
-  if test -n "${OIL_NINJA_VERBOSE:-}"; then
+  if test -n "${OILS_CXX_VERBOSE:-}"; then
     echo '__' "$cxx" $flags -o "$out" -c "$in" >&2
   fi
 
@@ -299,7 +306,7 @@ link() {
     prefix="benchmarks/time_.py --tsv --out $TIME_TSV_OUT --append --rusage --field link --field $out --"
   fi
 
-  if test -n "${OIL_NINJA_VERBOSE:-}"; then
+  if test -n "${OILS_CXX_VERBOSE:-}"; then
     echo "__ $prefix $cxx -o $out $@ $link_flags" >&2
   fi
   # IMPORTANT: Flags like -ltcmalloc have to come AFTER objects!  Weird but
@@ -322,7 +329,7 @@ compile_and_link() {
 
   setglobal_cxx $compiler
 
-  if test -n "${OIL_NINJA_VERBOSE:-}"; then
+  if test -n "${OILS_CXX_VERBOSE:-}"; then
     echo "__ $cxx -o $out $flags $@ $link_flags" >&2
   fi