Simplify call to flint's configure script (autotools)

It no longer supports the LIB_DIRS variable but it does support CPPFLAGS and
LDFLAGS, so there's no reason to define our own CONFIGURECMD variable.
Instead, we can use the default one and just define PRECONFIGURE and add a
couple things CONFIGOPTIONS.

[ci skip]

M2/libraries/flint/Makefile.in

@@ -15,12 +15,9 @@ CONFIGOPTIONS += --enable-assert
 CFLAGS += -O0 -fno-unroll-loops 
 endif
 CFLAGS += -std=c90 -pedantic-errors
-# the flint configure script does not accept CPPFLAGS
-# CONFIGURECMD =  LIB_DIRS=$(LIBRARIESDIR)/lib ./configure  --with-gc --with-blas --disable-tls
-CONFIGURECMD =  ./bootstrap.sh && \
-	LIB_DIRS=$(LIBRARIESDIR)/lib ./configure --without-blas  \
-			--prefix='$(PREFIX)' --disable-shared CC='$(CC)' \
-			CFLAGS='$(CFLAGS) $(CPPFLAGS)'
+
+PRECONFIGURE = ./bootstrap.sh
+CONFIGOPTIONS += --without-blas --disable-shared
 
 ifneq ($(VERBOSE),)
 BUILDOPTIONS += AT= QUIET_CC= QUIET_CXX= QUIET_AR=